By this we can be sure that we have come to know Him: if we keep His commandments.
Keeping God's commandments shows that believers have come to know Him.
This verse introduces obedience as a test of genuine knowledge of God.
1My little children, I am writing these things to you so that you will not sin. But if anyone does sin, we have an advocate before the Father — Jesus Christ, the Righteous One. 2He Himself is the atoning sacrifice for our sins, and not only for ours but also for the sins of the whole world. 3By this we can be sure that we have come to know Him: if we keep His commandments. 4If anyone says, “I know Him,” but does not keep His commandments, he is a liar, and the truth is not in him. 5But if anyone keeps His word, the love of God has been truly perfected in him. By this we know that we are in Him: 6Whoever claims to abide in Him must walk as Jesus walked.