Love in truth and truth in love, is the character of the book.
V.4 We have the truth, for God’s Word is truth. But it is quite another thing to be walking in the truth (living the truth). The Apostle John rejoiced greatly when he saw this in the children of this woman.
V.5-6 Walking in truth will show itself in love for other believers.
V.7 Walking in truth will protect us from deceivers.
V.9-11 Truth is centered in Christ. He is truth (John 14:66Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me. (John 14:6)). He is God become Man. This is the meaning of the “doctrine of Christ.” The lady was to keep outside her house all that was not according to this doctrine. The sins of those who did not hold this would be transferred to her if she allowed those into her house and greeted them. If we think we can win Mormons or Jehovah Witnesses to Christ by allowing them into our house for a discussion, we are disobedient to God’s Word. Both deny that Jesus is the divine Son of God.