In Against Heresies, book 3, chapter 2, Irenaeus writes "When, however, they are confuted from the Scriptures, they turn round and accuse these same Scriptures, as if they were not correct, nor of authority, and [assert] that they are ambiguous, and that the truth cannot be extracted from them by those who are ignorant of tradition. For [they allege] that the truth was not delivered by means of written documents, but by word of mouth".
Over 1800 years later, Satan's tactics have not changed. Heretics, incapable of withstanding the test of God's word (1 Thessalonians 5:21) when it is wielded by those who search the Scriptures daily in search of the truth (Acts 17:11), will always appeal to other sources. These sources may give the strong illusion of authority. Tradition, history, these are things many people tend to respect.
And of course, it's not that there's not merit to this. You can't just come along one day and invent your own version of Christianity. We want to be connected to the early Church, especially those who knew Jesus and the Apostles personally. These, of course, know more about the faith than we do. Thus, heretics reason, if they can make it seem like their view is the historical one, that automatically discredits everyone else.
Due to the relative ignorance of the modern Church regarding the early Church, this isn't even very difficult to do. These days, so many Christians are "too busy" to study even their Bibles, so of course they're not going to read through everything else besides. But a devoted heretic has no qualms about doing so. Thus, they can cite the so-called Church "Fathers" left and right, either showing that they believed a certain thing, or at least making it appear so.
Now, I would consider it a fairly reliable rule that whenever someone says "the early Church did not believe...", many in the early Church absolutely believed whatever follows. "No one in the early Church believed in Eternal Security". Well, it seems fairly well developed in Augustine's writings... "No one in the early Church taught Credobaptism". Except, apparently, Tertullian.... This same Tertullian taught a very "Protestant" sounding interpretation of John 6 about 1200 years before Zwingli supposedly made it up.
The examples are endless, and the only reason heretics ever get away with claiming the early Church never taught anything other than what they believe is because most Christians don't study beyond their Bible.
And they don't have to.
See, we can go on for quite some time arguing about who believed what in the early Church, but one thing that is beyond reasonable dispute is that if the Bible teaches a thing, the early Church not only taught it, but they were right to teach it. From this same Bible, we also see that quite early on, many heresies existed. In fact, the New Testament is made up, in part, of divinely inspired corrections for the early Church. The Judaisers were running around saying circumcision is necessary for salvation, God wasn't very happy with this, so Galatians was written. The Gnostics were rampaging through the land claiming Jesus was merely a phantom, this displeased God, so 1 John was written. Sexual immorality was a major problem in the early Church, so Paul was quite vocal with the Corinthians.
Now, in the modern day, while there are still Gnostics, they are complete non-players in the theological world. But for a moment, let's suppose they were. They would have historicity. There were Gnostics in the first century. There were Gnostics when Rome fell. There were Gnostics during the Great Schism. There were Gnostics during the Reformation. There are still Gnostics today.
Suppose there were Councils convened in which the Gnostics emerged victorious. Imagine, if you will, the Church "Fathers" had Gnostic views, and Gnostic influences. Even as the case is, it is probably possible to make a Church "Father" sound Gnostic. Well, first of all, the existence of Gnostics does not rule out the existence of Christians. You could find 10, 20, even thousands of writings by Gnostics, this does not mean no one in history rejected Gnosticism. It just means whoever you found was a Gnostic.
But more importantly, Scripture wins by default. A key question to ask any heretic is a simple true or false: "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." Now, if they say false, that opens a whole new can of worms. But almost no one, claiming to be Christian, will deny these words from 2 Timothy 3:16-17.
Now, because of this simple fact, Scripture actually trumps history. Whether the history is real or imagined, the One God carries more authority than every last human being who has ever lived, or will ever live. So what if Clement of Rome disagrees with the Bible? What if Ignatius of Antioch contradicts the Scriptures? What if Augustine of Hippo misinterprets the New Testament? What if Tertullian of Carthage messes up on the Old Testament? As God is greater than man, so is His word greater than those who expound upon it. Therefore, we can even disagree with "the early Church".
Now of course, I use that phrase in air quotes simply because, as a Christian, I believe my beliefs are quite in line with the teachings of the early Church. Why? Simply because I study the same Scriptures. Now of course, that doesn't mean everything I believe is pinpoint accurate. I make mistakes. I learn. So did the Church "Fathers". Augustine, by far the most well-known, well-cited, and well-respected Church "Father", even wrote a work entitled "The Retractions", in which he revokes a number of his past teachings.
So not even "the early Church" was perfect. But Scripture is. You may well find early Church quotes that promote heretical ideas, and you may be hard pressed to find examples of the early Church teaching sound doctrine. But sound doctrine it remains. Because it comes from God. The pages of Scripture are the absolute best place to find not only the traditional teachings of the early Church, but also the unassailable teachings of Almighty God!
Ultimately, therefore, whenever a heretic claims the early Church did not believe something, or explicitly taught their heresy, you can dismiss it even without knowing any extant writings. If you are so inclined, you may study Church history. In so doing, you will find many doctrines that the early Church supposedly did not believe. I myself find I am often able to quote Church "Fathers" without telling my opponent I'm quoting them, only to reveal where the quote came from after I am accused of heresy. This is a strategy I like to call the "silent quote". But in the end, you do not need to sift through endless pages of man's literature. The Scriptures are all we will ever need to show that our beliefs are correct, for these are the words of God, and not even His Church may overrule Him.