A.B. Simpson emphasizes the necessity of a tender heart in fostering unity among believers, recounting a situation where two co-workers were at odds despite one being technically right. He highlights that truth, when not delivered with love and grace, can lead to division and hinder God's work. Simpson points out that true oneness, as prayed for by Jesus, requires individuals to take responsibility for their own sins rather than blame others. He illustrates this with the example of the prodigal son and the thief on the cross, both of whom acknowledged their failures. Ultimately, maintaining a tender heart is essential for achieving the unity that Jesus desires among His followers.