i.e. If he truly believe, he hath the effectual impress of this testimony on his own soul; if not, he gives God the lie, as we do to any one whose testimony we believe not. See Poole on "1 Thessalonians 3:33".
Related Commentaries of 1 John 5:10
1 John 5:10
10 He that believeth on the Son of God hath the witness in himself: he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son.