Scripture says it has the final say on doctrines:
2Ti 3:15 And that from a child
thou hast known the holy scriptures, which are able to make thee wise unto salvation through faith which is in Christ Jesus.
2Ti 3:16 All scripture is given by
inspiration of God, and is profitable
for doctrine, for reproof, for correction, for instruction in righteousness:
2Ti 3:17 That the man of God may be perfect, throughly furnished unto all good works.
And says it’s our responsibility to study the scriptures for ourselves, and learn the truth:
2Ti 2:15 Study to shew YOURSELF approved unto God, a workman that needeth not to be ashamed,
rightly dividing the word of truth.
And the Beareans are commended and called honorable, for not just taking the apostle Paul’s word for it, but searched the scriptures to see if he was right:
Act 17:11 These were more noble than those in Thessalonica, in that they received the word with all readiness of mind,
and searched the scriptures daily, whether those things were so.
And Jesus affirmed all the above:
Joh 5:39 Search the scriptures; for in them ye think ye have eternal life: and they are they which testify of me.
Mat 22:29 Jesus answered and said unto them,
Ye do err, not knowing the scriptures, nor the power of God.
What’s the ultimate source of truth for Protestants?
Scripture.
Maranatha