In our day, to call Theistic Evolution a heresy is considered divisive, intolerant, and hateful. You can say what you want about "young" Earth Creationism, from claiming it was made up by the 7th Day Adventists to saying anyone who believes in it is an uneducated moron with no grasp on the concept of hermeneutics, but if you say Theistic Evolution is a heresy, you may as well have just cussed out a thousand old ladies.
But as the proverb says, "The wicked flee when no one pursues, But the righteous are bold as a lion." (Proverbs 28:1). As Christians, we need not fear calling out heresy wherever it stands, even if it is cradled in the arms of a fellow Christian.
And to be clear, I do mean Christian. The Christian faith is amazingly permissive in just how little you need to have right in order to be found within its ranks. A man who converted yesterday, after hearing a 5 minute pitch from a street preacher, is just as much a Christian, knowing next to nothing about Christianity, as a veteran pastor who spent his entire adult life studying and preaching Scripture.
Ironically, Christianity even has a fair few warnings about knowing too much. Knowledge puffs up, but love edifies (1 Corinthians 8:1). So what do we do with someone who, having been raised in a pagan culture, doesn't fully understand that idols are nothing, and the food pagans sacrifice to them is nothing special? Well, according to that same passage, we don't eat the food, for sake of their conscience, not for ours.
A Theistic Evolutionist is little different. Just as a pagan doesn't necessarily understand that an idol has no power, Theistic Evolutionists likely do not understand that Evolution is yet another false religion, propagated by the world, influenced by the god of this age (2 Corinthians 4:4), with the explicit intention of wiping out the Christian foundations of science.
So of course we can still count them as Christian. To be a Christian requires very little. One must repent of sin (which does not mean sinless perfectionism), confess Jesus as Lord (so yes, the Trinity is essential, otherwise you're not actually confessing Jesus as Lord), and believe in your heart God raised Him from the dead. As long as you know, believe, and respond to the Gospel, you can be assured of salvation, no matter what sort of strange, alien teachings you bring along with it.
But those strange teachings can still be called "heresy". There is a lot of confusion in the Church about what a heresy actually is. If you say "that is heresy", less educated Christians often hear "you're not Christian". But in truth, as I just said, Christianity is simplistic enough that one can believe some very strange, even immoral things, and still be a Christian.
Standard definitions of heresy don't really help us here. One might, based on certain dictionaries, or dodgy websites like Wikipedia, define heresy along the lines of "teachings that go against the orthodox teachings of the Church". But the problem with this definition is we then have to ask, which Church? Using this definition, you can simply declare your Church to be the one true Church, teach "all our hope do we repose on the most blessed virgin", and then if you say Mary played no greater role in our salvation than being the chosen vessel through whom the Lord entered our world, you're a heretic.
The next step would be to go to councils and see what they say. But this brings up more problems than before. Again, which councils? Under the Council of Trent, the Apostle Paul would be declared anathema! So already this can't be a good place to look. But then even good councils, like Nicaea, should only be viewed as descriptive, not prescriptive. When Arianism was officially declared a heresy, it was because Arianism was already heretical. Had that same council declared the Trinity heretical instead, it would have been the council that was teaching heresy.
So this isn't a good standard either. But then, what is heresy, and how do we know? According to Scripture itself, "All Scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness, that the man of God may be complete, thoroughly equipped for every good work." (2 Timothy 3:16-17). This is one of many Scriptures that tells us Scripture alone is the sole standard of faith. What the Bible says is true, we must believe. What the Bible says is false, we must reject. What the Bible leaves open ended, we must either leave open ended, or, as is the case for non-spiritual things (how to make a car, for example), discover for ourselves. Heresy, therefore, is most easily defined as a belief that is not reasonably compatible with Scripture.
When explaining the concept of heresies, I often like to use two doctrinal examples: Calvinism, and Theistic Evolution. Calvinism is not a heresy, or at the very least I cannot bring myself to identify it that way. Having studied Scripture, listened to Calvinists present and defend their view, and even at one point being drawn to Calvinism (though I never accepted it), I completely understand how, studying Scripture alone, one might draw Calvinist conclusions. I do not believe Calvinism is true, and I am now a firmly convinced Molinist, but "is it true?" and "can one reasonably conclude, based on Scripture, that it is true?" are two completely different questions. The sovereignty of God and responsibility of man are actually two very complex topics that I don't even think we will fully understand until we get to the Kingdom (1 Corinthians 13:9-13). Thus, to me, the main problem with Calvinism is it seeks to claim that knowledge now, by leaning heavily on one aspect of Scripture, while ignoring other aspects. This, of course, is still a major no-no, and can be symptomatic of perdition, but is not, in and of itself, heretical.
When we use the same standard to analyse Theistic Evolution, however, we find it sorely lacking. We can of course begin with the simple fact that the Church was virtually unanimous on this topic, right the way up until the 1700s at the earliest. This does not mean there were no disagreements on anything relating to origins. There was, for example, a strange belief that the 6 days of creation were only one day. However, millions of years, a local flood, and an allegorical Adam, are completely absent from any historical writings. At least, any within the Church. Instead, everyone, from Irenaeus, right the way up to Isaac Newton, believed the Earth was roughly 6,000 - 10,000 years old, with some even suggesting that the Earth will end exactly 6,000 years after it was created.
And it's not just the Church. The Jews had their ducks in a row on this topic, too. Josephus, for example, taught that Moses says the Earth was created in 6 days, that Isaac died 3,833 years after the creation, and reiterates Moses' creation account (which he binds to the Sabbath, as Scripture does).
Even long before Josephus, the Jews held a very straightforward interpretation of Genesis, as is shown by the extra works they wrote. Tobit, for example, contains a prayer said by Tobiah and Sarah, in which they reference God's creation of Eve as a helpmeet for Adam, and that the human race is descended from them.
We see, then, that "Young" Earth Creationism has been the dominant interpretation of Scripture since long before Scripture was completed. The New Testament did not exist when "Young" Earth Creationism saturated the believing world. By contrast, no one sought to merge Evolution, the Big Bang, and millions of years with Scripture until the atheists told them to. There was no magical explosion in the 2nd century. There were no monkey men 1,000 years ago. Even 500 years ago, if you started running around saying "God made the Earth 100,000 years ago", believers in every denomination would look at you as if you'd just declared yourself the direct descendant of the angel Gabriel. It just wasn't there.
And of course it shouldn't be. To begin with, in the first 8 pages of Scripture alone, we see the entire process of creation, the fall, the flood, and the dispersal of nations. This, Theistic Evolutionists tell us, is not supposed to be taken "literally", but in so doing, they are confessing that this is what it literally says, tipping their hand a little too low.
See, one of the most basic rules of studying Scripture, and indeed studying anything, is that unless good reason is given, we default to the most literal interpretation. We don't do this in the Psalms, because the very word "Psalm" means hymn. It's poetry, and so of course we can take it figuratively. Proverbs, likewise, is a collection of Hebrew sayings; figures of speech whose literal meaning must be discerned via intelligent thought and diligent study.
Interestingly, even metaphors have some very literal applications, and can be appropriately used within literal texts. If, for example, I was to say "I'm going to get some work done while the sun is out", I literally mean I want to get some work done, but I don't literally mean while the sun is out, as if the sun ever actually goes inside anything. The Bible actually uses similar metaphors, describing the sun and moon as lights in the sky. The moon doesn't even have its own source of light, it merely reflects the light of the sun. But why should God take extra steps to describe astronomy in painstaking detail, just to accommodate a few stubborn rebels in the 19th - 21st centuries? Jews and Christians alike have understood Genesis for 2,000 years. Even today, here I am, a single representative of a great many millions of Christians who affirm the truth of the Genesis creation account. Thus, to demand God literally change His account of origins for your sake is really just arrogant.
And that arrogance is particularly problematic. See, although the heresy of Theistic Evolution is not an automatic death sentence, it can be symptomatic of a greater problem. Scripture is replete with testimony that faith and works are inextricably linked (e.g. James 2:17, 20, 26), that as man thinks, so is he in his heart (Proverbs 23:7), that heresies are a work of the flesh (Galatians 5:19-20), and that we will know Christ's disciples by their fruits (Matthew 7:16-20).
Now again, being a Christian does not make one instantly perfect (and indeed, according to 1 John 1:8-10, denying our sin proves we are self-deceived), and so one foul belief is not automatically proof that one is not a Christian. But heresies breed, and errors travel in packs, whereas saving faith in God sanctifies us, and diligent Bible study moulds us in Christ's image. Now, if as a man thinks, so is he in his heart, then what is he in his heart if he thinks he is a product of chance, millions of years of monkey sex, and a little bit of help from God, who sits in Heaven declaring cancer "very good"? If you believe God is such a psychopath who enjoys watching violence, pain, and suffering, even when there is no sin to merit them, then why would you not act like the glorified ape you think you are?
This is why Theistic Evolutionists tend to be more Liberal in other areas, too. Abortion, homosexuality, and the trans agenda, are common immoral practices they tend to support. And many of them even flat out reject the resurrection, which is a genuinely fatal heresy. You are not a Christian, your sins are retained, and you will be cast into Hell on judgement day, if you do not believe that Jesus Christ is the risen Lord.
Now of course, not all Theistic Evolutionists are quite that radical. But I do have to ask, what is going on in your heart if such a simple Biblical concept is so repugnant to you? What motivates you to believe God loves death and violence, rather than that He created a very good world that fell? What pleasure do you receive from the idea that millions of years before the first sin, cancer was eating the bones of dinosaurs who lived their lives in agonising pain? What reward does the lie that the bonobo you see in the zoo is actually your cousin bring you?
I understand why atheists might accept Evolution. But for a Christian, there are only really two reasons: An extreme lack of knowledge, requiring some of the poorest study habits the human race has ever exhibited, or some kind of heart issue that might allow some faith in Christ, but also keeps us running from Him.
Faith in Evolution is such a fruit of the world that even they cannot adequately discern a Theistic Evolutionist from themselves. By contrast, if you so much as hint at Intelligent Design, their alarm bells immediately go off: "That's a Christian! Eww, keep your religion away from me!" My brethren, God knew what He was doing, both when He created the heavens and the earth, and when He told Moses how He did it. It is not our job to decide He's wrong, or to follow those who claim He is wrong. It is our job to trust Him, even when the devil tells us not to. After all, look what happened the first time he lead us astray...