The important word "truth" is a key word in the apostle John's vocabulary, occurring more in his writings than in those of any other New Testament author. In the one-chapter epistle of 3 John, it occurs six times.
He addressed his epistle to "Gaius, whom I love in the truth" (v. 1). He rejoiced over "the truth that is in thee" (v. 3). He urged his readers to be "fellowhelpers to the truth" (v. 8) and commended Demetrius, who had a good report "of the truth itself" (v. 12). But he was especially pleased to hear that his beloved disciple, Gaius, was walking in the truth (v. 3).
Every godly Christian parent, pastor, teacher, or personal soul winner knows exactly how John felt. There is no greater joy to such mature believers than to know that their children--whether physical or spiritual children--are sound and growing in Christian faith and practice, understanding, and believing God's truth, obeying His truth, living the truth, walking in the truth. But there is great sadness when such a one begins to "turn away their ears from the truth" (2 Timothy 4:4) and to walk again in step with the fables of the world. When they "walk in darkness" and "do not the truth" (1 John 1:6), this brings pain and sadness to those who have loved them and tried to lead them in the light of God's truth.
And what is the truth? John's very first mention of truth was in relation to Jesus Christ, whose glory he had beheld in His incarnation as "full of grace and truth" (John 1:14). In fact, Christ Himself claimed, "I am . . . the truth" (John 14:6). Then He also said, "Thy word is truth" (John 17:17). "And it is the Spirit that beareth witness, because the Spirit is truth" (1 John 5:6). To walk in the truth, bringing joy to the Father, is to believe on Christ, and then to trust and obey His word as illumined by the witnessing Spirit. HMM