Here's just one relevant quote among many from today's radio program:
When a person departs from the doctrine that the apostles had taught, Paul sees this as a greater threat to unity than the disunity caused by avoiding such people. If we say: How can that be? How can dividing from a false teacher who rises up in the church promote unity in the church? The answer is that the only unity that counts for unity in the church is rooted in a common apostolic teaching. Isolating false teachers—avoiding them—is Paul’s strategy for preserving unity that is based on true teaching.You can see the whole manuscript here or download an MP3 here.
The bottom line? Unity is a sham unless it's unity around truth. Piper discusses loving people and loving truth, purity for the sake of unity, a defined body of doctrine, and truth-based division for the sake of truth-based unity.