Steve Ray has a list of 35 loaded Questions for "Bible Christians" (quotation marks his)(link to the whole list). This is number 32/35. I'm trying to provide the answers in a common format, for easy reference.
32) If the Bible is the only foundation and basis of Christian truth, why does the Bible itself say that the Church is the pillar and foundation of truth (1 Tim. 3:15)?
The question equivocates between the Bible's role as our source and standard of truth and the church's role as defender and promoter of truth. The two are different, even if they can be expressed using the same words (in different senses).
1) The Bible is not the only source of truth for Christians. We accept both General Revelation (also called the "light of nature") and Special Revelation (of which Scripture is the foremost example, but which also encompasses the prophecies of all true prophets).
2) The church's purpose is to be a supporter and defender of the truth. That is its purpose and it's role. It does not always perform that role well.
3) Roman apologists get so excited by this aspirational goal of the church that they miss Paul's stated reason for writing the letter explained in the immediate context:
1 Timothy 3:14-35
These things write I unto thee, hoping to come unto thee shortly: but if I tarry long, that thou mayest know how thou oughtest to behave thyself in the house of God, which is the church of the living God, the pillar and ground of the truth.
Notice that Paul says that the reason he is writing is to provide information to Timothy that Timothy would not have if Paul were to tarry (i.e. delay coming), even though Timothy is "in the house of God" even while Paul is writing to him. In other words the Church that is the "pillar and ground of the truth" is unable to supply Timothy with the instruction that Paul is providing, and consequently Paul has written to Timothy. In short, Scripture is needed because "the church" is not enough for Timothy. So, appeal to this verse backfires on the advocate of Romanism.