What's the significance of the extrabiblical sources?

We need to keep in mind that the significance of extrabiblical sources varies, and can vary widely, from one context to another. On a subject like eternal security, which I've been addressing a lot in recent months, we're in a context in which the Biblical sources provide us with a large amount of information. It's not as though eternal security is some minor issue that never came up or only came up on rare occasions in the Biblical record. It's not something with as little significance as what year Isaiah died or how many times Paul visited a particular city. The potential to lose justification has existed since the time of Adam and Eve, instead of being something that only came up toward the end of the Biblical era or afterward. The Bible provides us with relevant information in dozens of documents from dozens of authors over more than a thousand years. A supposed lack of clarity in one Biblical source can be resolved by consulting another passage or group of passages elsewhere in that source or by consulting one or more other Biblical sources. The nature of eternal security is such that our dependence on extrabiblical sources is much less in that context than it is on other issues.

Something like a universal or nearly universal absence of or opposition to eternal security among the extrabiblical sources would give us reason to reconsider our view on the subject, but any conclusion we'd reach would still have to interact with the large amount of Biblical data we have on the topic. But there isn't a universal or nearly universal absence of or opposition to eternal security among the extrabiblical sources, as I've demonstrated in my posts on the subject. Since eternal security is addressed so much in scripture and is neither universally nor almost universally absent or contradicted in the extrabiblical sources, we have a situation in which the extrabiblical sources are less significant accordingly.

Whether the topic is eternal security or something else, we need to remember what's involved when people refer to something like "the Bible" or "scripture". There's a sense in which only one source is involved, but there's also a sense in which there isn't. We could similarly refer to the church fathers collectively as "the church fathers" or refer to medieval sources collectively as "medieval sources", for example. But the Bible, like those other collections of documents, consists of many sources who wrote in many contexts. Extrabiblical sources have some value in assisting us in interpreting the Biblical documents, and some people underestimate the value of those extrabiblical sources (because of ignorance, laziness, dishonesty, or whatever other reason), but there's also a danger of overestimating them. And the level of significance they have varies from one context to another.