I'm writing you

14 I hope to come to you soon, but I am writing these things to you so that, 15 if I delay, you may know how one ought to behave in the household of God, which is the church of the living God, a pillar and buttress of the truth (1 Tim 3:14-15).

i) V15 is a traditional Catholic prooftext, much used by contemporary Catholic apologetics. A basic problem with their appeal is quoting the verse out of context. A pitfall of chapter and verse division is that Christians sometimes read a particular verse while failing to place that verse in the flow of argument. They don't consider what comes before or after. 

Catholic apologists say, "See, Paul doesn't say "Scripture" is a pillar of truth, but "the Church". Yet they completely ignore the preceding verse. Paul is directing Timothy to what he wrote.  Look at what I just wrote you!

ii) Moreover, he wrote Timothy so that Timothy would know how to conduct himself in church, based on Paul's written instructions. If, however, the church is the source of truth, then that's superfluous. Yet Paul points Timothy to Paul's explicit, written directives. That's the benchmark. 

iii) Syntactically, v14 refers back to the preceding section (2:1-3:13). But the principle extends to the rest of the letter. Since Paul can't instruct Timothy and the congregation in person, the letter is a stand-in, which serves that purpose.

iv) By Paul's own admission, his letter takes the place of Paul's face-to-face teaching. Catholic apologists claim we need a "living voice". An infallible interpreter. Yet the function of an apostolic letter is to instruct the faithful in the apostle's absence (cf. 2 Cor 13:10).

It would be insubordinate to say, that's only a text, so we can't know what Paul really meant. That's why we have apostolic successors like Timothy, to infallibly expound the deposit of faith.

Yet Paul takes for granted that his written instructions should suffice in his absence. And even if we anachronistically classify Timothy as a bishop, Timothy has no independent authority. Timothy can't say, by virtue of his "office", how Christians are supposed to behave in church. That's based, not on Timothy's teaching authority, but on Paul's teaching authority, in written form. Timothy simply transmits what he was taught by Paul. There's nothing here about the necessity of an infallible teaching office to interpret the deposit of faith, even though Paul is nearing the end of his career. He will soon pass from the scene. He will have to hand off the work to the next Christian generation. 

v) Even if Timothy received oral instruction from Paul in the past, the letter is an aid to memory.