“And we know that the Son of God is come, and hath given us an understanding, that we may know him that is true, and we are in him that is true, even in his Son Jesus Christ. This is the true God, and eternal life.”
King James Version (KJV)
5:20 And we know - By all these infallible proofs. That the Son of God is come - Into the world. And he hath given us a spiritual understanding, that we may know him, the true one - The faithful and true witness. And we are in the true one - As branches in the vine, even in Jesus Christ, the eternal Son of God. This Jesus is the only living and true God, together with the father and the Spirit, and the original fountain of eternal life. So the beginning and the end of the epistle agree.
1Jo 5:20 And we know that the Son of God is come. Not only by testimony from men but by God's witness. We have life in Christ, and Christ dwells in us. This is the true God, and eternal life. In Christ the true God is revealed to us, and in him we have eternal life.