One argument attempting to defend Theistic Evolution is what I like to call the "red letter defence". A growing number of Christians take the title "Christ-ianity" so seriously that if Jesus did not physically say something with His human mouth, typically indicated in modern Bibles by red text (hence the name "red letter defence"), it is not an issue that is important to Christianity. Thus, when applied to the origins debate, the argument is that Jesus never said anything about Evolution, therefore there is no official Christian position to take on the subject.
There are three major problems with this defence. The first is that those who make it are rarely consistent with it. "Jesus didn't say it, therefore it doesn't matter" is used by people who believe a lot of things matter that Jesus didn't say. In fact, these same people will often criticise Creationists for allegedly making Christians look bad. I can assure you, Jesus never said "you can allegorise the creation account in Genesis if atheists start calling you stupid for taking it as historical narrative". Read the Bible forwards, backwards, and upside down, you won't find Jesus saying anything remotely negative about Creationism, or positive about Evolution.
Second, it's contradictory, because the red letters affirm the black. Time and time again, Jesus appealed to, and even directly quoted scripture, saying it really is the inerrant, unbreakable word of God. In other words, sometimes the black letters can turn red depending on the page you're reading from. More to the point, Jesus actually went so far as to say "if you believed Moses (the author of Genesis), you would have believed me" (John 5:46, brackets mine). The logical reverse of that is "if you believe me, believe Moses". He also said things like "the Scripture cannot be broken" (John 10:35), "till heaven and earth pass away, one jot or one tittle will by no means pass from the law till all is fulfilled" (Matthew 5:18) "Whoever therefore breaks one of the least of these commandments, and teaches men so, shall be called least in the kingdom of heaven..." (Matthew 5:19) and a host of other statements that affirm the truth of scripture, and both condemn and even mock those who do not believe it. You simply cannot afford to dismiss a single word of the Old Testament if you believe Jesus.
But the final flaw in this argument is that Jesus was far from silent on the topic of origins. He actually appealed to the origins account in Genesis as a foundation for other doctrines. For example, when questioned on divorce, Jesus said that God made man male and female from the beginning of creation (Mark 10:6). Yet, by the Evolutionary timeline, first of all, male and female came long before man, and second of all, there is no sensible way in which we could claim man has been around since the beginning of creation.
By contrast, reading Genesis as a historical narrative, we see that mankind was, indeed, created male and female from the beginning of creation. Within the very first week, there was already a living human couple on the Earth.
It gets worse, however. According to Genesis 1, after God created man male and female (I repeat, in the beginning of creation), He saw all that He had made, and He called it "very good". Deuteronomy 32:4 even reinforces this point by saying that God's works, which would include the heavens and the earth, are perfect. Let me ask you a rhetorical question: Which is better? Literally billions of years of suffering, pitiless brutality, and death, or a world in which all life lives peacefully, with the universe itself being designed to support it indefinitely?
I'll give you a clue: God actually tells us why we no longer see the latter. See, God gave the original human couple a very simple command: Do not eat from the tree of knowledge of good and evil. Within the command, He implemented a warning. If the command was broken, death would follow. But then, Satan crept in, tempted Adam and Eve to eat from the tree, and surprise surprise, death entered the world (Romans 5:12). All of creation was cursed (Romans 8:22), including the very ground (Genesis 3:17), and all the animals (Genesis 3:14).
With all of this in mind, we're actually talking about a Gospel issue here. Not in the sense that Theistic Evolutionists are not saved. That depends on whether they believe in the solution, i.e. the death and resurrection of the Lord Jesus Christ. But in the sense that Theistic Evolutionists must necessarily deny the original problem, we are talking about a Gospel issue.
Thinking of the Bible as a house in which the Gospel lives, Genesis is the foundation upon which it stands. If you remove Genesis, you still have the rest of the house, and the Gospel still lives inside it. But what happens when the ground begins to shake? That's when the walls start to crumble, and the roof starts to collapse, and the windows shatter. As the Psalmist asks, "if the foundations are destroyed, what can the righteous do?" (Psalm 11:3).
A Theistic Evolutionist may well be saved. However, if a man is justified by works (James 2:24), it is not a stretch to question the salvation of one who promotes such a detestable heresy. To those who are saved, I say this: Stop destroying the Genesis foundation, lest you condemn others. To those who are not saved, I say this: Don't be surprised when, having destroyed your own foundation, your faith begins to fall, and you head towards apostasy. When you start supporting Evolution, you have taken the first step to embracing the atheistic worldview that produced it. If you trust God, build on the rock He has provided: all of His words.