If the Bible is the only foundation and basis of Christian truth, why does the Bible itself say that the Church is the foundation and basis of Christian truth (1 Tim. 3:15)?
1 Timothy 3:15 is a somewhat ambiguous passage, with different possible interpretations. It may well be that Paul was talking about the church as the "pillar and support" of the truth, or it may be that he was talking about God himself. The first is the more intuitive reading, though the second is also possible.
Assuming Paul was talking about the church (which it seems to me that he was), it is still important to recognize what he did and did not say about the church. The first thing worth noting is that he did not state that the church was the "only" foundation and basis (or only "pillar and support").
Second, it is important to note that Paul used an architectural metaphor here. When he said that the church was the "foundation and basis" or "pillar and support," he was not saying that the church determines truth or that truth can only exist if it is built upon the church. After all, truth existed before creation, so it cannot be dependent upon anything in creation. Rather, he is saying that the church holds up and supports the truth, much as a building might be held up and supported by its foundation, pillars, base, bulwark, etc. The picture is of the church as defender and exhibitor of the truth, not as the determiner of truth or as prerequisite to truth. Paul's point was that God has delivered his true revelation to the church, and it is the church's job to protect and proclaim that truth.
Third, Paul did not say that the church could not fail to do its job. That fact that the church is the pillar and support of the truth does not imply that the church will always defend the truth, or that it will always proclaim the truth. In fact, throughout history the church has often failed both to defend and to proclaim the truth, and it always fails to do this to some degree. Paul wanted Timothy to understand the important role the church had been given because Timothy was an officer in the church who was charged to uphold the truth (cf. 1 Tim. 6:20), specifically in this case by making sure that the church's conduct was appropriate. Timothy was to guard the reputation of the church in order that the church's witness to the world would not be compromised. If Timothy had failed in this regard, which was a real possibility, the pillar and support of the church would have faltered -- the truth would have been mistreated and the church's witness to the world would have been damaged.
