I John 3:19 “And hereby we know that we are of the truth, and shall assure our hearts before him.”
Those whose “love” is “not” “in word, neither in tongue; but in deed and in truth” (John 3:18) “know that” they “are of the truth” because the author of such sincere “love” is God and not man. This sincere “love” does not make us “of the truth”; but, it is confirming evidence “that we are of the truth”; that is, this sincere “love” is the fruit, product of being “of the truth”. The “truth”, in this context is “that we have passed from death unto life” (I John 3:14); that is, “passed from” spiritual “death unto life” in Jesus Christ our Beloved Savior and Lord. Paul explains it this way, “Even when we were dead in sins, hath quickened us together with Christ, (by grace ye are saved;)” (Ephesians 2:5) and the Lord called it, being “born again” (John 3:1-8). And Paul wonderfully ties together our salvation to Eternal Heaven and salvation to spiritual life (Ephesians Chapters 1 and 2) and then declares that “For by grace are ye saved through faith; and that not of yourselves: it is the gift of God:” (Ephesians 2:8). The “truth” is that the same power and will of God that “saved” us to Eternal Heaven has also “saved” us from spiritual death. To have this blessed “love” in our mind’s heart is to have sure evidence that we are the Children of God, and acting upon this “love” “shall assure our hearts before him”; that is, give us comforting assurance that God is our Father, Jesus is our Savior, and the Holy Spirit is our Comforter! Thus, even in the times of our greatest trials, with this Blessed Assurance, “we know that if our earthly house of this tabernacle were dissolved, we have a building of God, an house not made with hands, eternal in the heavens” (II Corinthians 5:1)!