Adam Clarke on John 5:43

I am come in my Father's name - With all his influence and authority. Among the rabbins, it was essential to a teacher's credit that he should be able to support his doctrine by the authority of some eminent persons who had gone before. Hence the form, Coming in the name of another.

If another shall come in his own name - Having no Divine influence, and no other authority than his own, him ye will receive. That this was notoriously the case may appear from Josephus, Antiq. b. xviii. c. 14; Act 5:36, Act 5:37; Eusebius, Eccl. Hist. b. iv. c. 6. It is by the just judgment of God, that those who will not believe the truth of God shall be so given up as to believe the most absurd of lies. For an account of these false Christs, see the notes on Mat 24:5.