Why True Unity Flows From the Truth

The reason for this is that truth frees us from the control of Satan, the great deceiver and destroyer of unity: “you will know the truth, and the truth will set you free” (John 8:32; cf. 2 Tim. 2:24–26). Truth serves love, the bond of perfection. Paul prays for the Philippians that their “love [may] abound more and more, with knowledge and all discernment” (Phil. 1:9). Truth sanctifies, and so yields the righteousness whose fruit is peace: “Sanctify them in the truth; your word is truth” (John 17:17; cf. 2 Pet. 1:3, 5, 12).
For the sake of unity and peace, therefore, Paul labors to set the churches straight on numerous issues—including quite a few that do not in themselves involve heresy. He does not exclude controversy from his pastoral writing. And he does not limit his engagement in controversy to first-order doctrines, where heresy threatens. He is like a parent to his churches. Parents do not correct and discipline their children only for felonies. Good parents long for their children to grow up into all the kindness and courtesy of mature adulthood. And since the fabric of truth is seamless, Paul knows that letting minor strands continue to unravel can eventually rend the whole garment.
Thus Paul teaches that elders serve the church, on the one hand, by caring for the church without being pugnacious (1 Tim. 3:3, 5), and, on the other hand, by rebuking and correcting false teaching. “He must hold firm to the trustworthy word as taught, so that he may be able to give instruction in sound doctrine and also to rebuke those who contradict it” (Titus 1:9; cf. 1:13; 2:15; 1 Tim. 5:20). This is one of the main reasons we have the Scriptures: they are “profitable for teaching, for reproof, for correction, and for training in righteousness” (2 Tim. 3:16).

The Future of Justification, pg. 31 John Piper