How do we know the epistles are for all believers today? They were written to specific audiences in the cities of Rome, Corinth and Philippi etc. How do we know that those Epistles now apply to all believers? Yes, the epistle 2 Peter 3:16 speaks of some of Paul's epistles being hard to understand back then, but is there anything in the New Testament that would show that principle has applied ever since to his epistles, or that other epistles were going to apply to believers in the future?
I would suggest two good reasons to understand that Paul's epistles apply to all believers today:
- They are specifically called "scriptures" by Peter.
And account that the longsuffering of our Lord is salvation; even as our beloved brother Paul also according to the wisdom given unto him hath written unto you; As also in all his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction. (2 Peter 3:15-16)
- We know the scriptures are for our study, benefit, and salvation. Jesus said:
Search the scriptures; for in them ye think ye have eternal life: and they are they which testify of me. (John 5:39)