Multiple things.

1) Comparing the doctrine as defined in the original Protestant confessions to the scriptures.

For example, WCF 1.6 basically states that all things pertaining to a matter of salvation, faith, or life must be explicitly stated in scripture, or implied in such a way that you can deduce it by good and necessary inference.

Then realizing that, although scripture does speak of itself as being authoritative, binding, God-breathed, and so on, it never speaks of itself as being the only or even ultimate source for doctrine. So in that sense, it's a self-defeating doctrine based on it's own definition.

2) Performing a systematic study of early fathers and church history. Realizing that, although there was a high view of scripture in the early church, just as there is in the EO and RC churches today, references to scripture in the early church was almost entirely exclusive to the OT scriptures prior to Nicaea I, and the OT does not support sola scriptura. The utterances of prophets, for example, was considered an infallible source for doctrine, because it was God speaking through the prophet. The mode of communication was different (oral vs written), but it was still God-breathed, and didn't only become infallible or authoritative once written.

The early church was not sola scriptura. It viewed oral tradition as being on equal footing as the written word. It also viewed the consensus of bishops as being on equal footing with the written word. That is where the three-fold structure of the RCC comes from...Tradition + Scripture + Magesterium.

3) Studying scripture itself apart from the Protestant confessions and church history. For example, the Council of Jerusalem. It occurred sometime between 48-50AD. It wasn't recorded in the book of Acts until 80-90AD. Yet, the decree of the council (Gentiles did not need to first become Jew into order to become Christian) was a binding and authoritative decree the moment the council concluded. It didn't only become such 30-40 years later after Luke recorded the events in Acts. We know this because of Paul's epistles, the events in which occurred after the council and well prior to Acts being written in 80-90 AD. In other words, the council itself, and not the written record of the council, was the ultimate authority on that particular matter.

Same thing with circumcision in Genesis. Jewish males needing to be circumcised wasn't only an infallible and binding doctrine for the OT covenants once Moses wrote about it 2000 years after God communicated the rite to Abraham. It was binding immediately when God communicated it to Abraham, and continued to be binding as it was communicated orally down to the people in Moses' day.

4) Realizing that there was a period of around 16 years before a single word of the NT was written, and almost 30 years before a Gospel was written. Yet, the Gospel still authoritatively and infallibly spread far and wide in that period. Had not a single word of the NT been written, we would still have the Gospel today, just like the people in Moses' day still knew the story of creation more than 2000 years later and before a single word of it was written.

5) Finally, realizing that Jesus didn't leave us with the Bible and never told anyone to write anything down. Instead he gave us a church and then blessed that church with indefectability and infallibility. The bishops of the church then in turn carefully (usually) chose men to replace themselves with, laid hands on them, and charged them with continuing the mission.

This is what Irenaeus appealed to in his debates with the Gnostics. He appealed almost exclusively to apostolic succession, and the bishop lists kept in each NT church, because the Gnostics had the same scriptures as Irenaeus. So he had to appeal to another infallible authority in order to show them why they were wrong in their interpretations which were based on their reading of scripture.

Thats a lot and poorly written due to being at work and on my phone, but that's the gist of it.

ETA: it seems that a recent argument gaining traction in support of sola scriptura is the one James White used against Trent Horn in their debate on the topic earlier this month.

White basically admitted that sola scriptura was not possible to be true during the first century, because revelation was still being given. Then it was impractical to implement prior to Nicaea I, because the canon hadn't been discerned.

My problem with this is that the period of public divine revelation is precisely when I would expect God to communicate the single most important doctrine of the reformation to us, yet he didn't, as admitted by White in that debate. Another problem is that if it wasn't practical to implement prior to Nicaea I, you would need to show from scripture where it was prophecied that the doctrine would become true in XYZ time period after Nicaea I. That didn't happen either.

Then there's the issue of the canon itself. White seems to think that once the canon was discerned by the Church, sola scripture became a true doctrine. Yet, he disagrees with the canon. So the canon was a prerequisite to sola scriptura being able to be implemented, yet the canon was wrong until Luther corrected it in the 1500s. Just seems illogical to me.

This post was edited on 3/26/24 at 1:05 pm