John Gill Commentary John 5:43

"I am come in my Father`s name, and ye receive me not: if another shall come in his own name, him ye will receive." — John 5:43 (ASV)

I am come in my Father's name
Power and authority; by his consent, with his will, and according to a covenant with him: Christ came not of himself, of his own accord, by a separate power and will of his own, but was called, and sent, and came by mutual agreement; and brought his credentials with him, doing the works and miracles which his Father gave him to finish:

and you receive me not ;
notwithstanding this they rejected him as the Messiah, and would not receive him as such; yea, traduced him as an impostor, and a deceiver:

if another shall come in his own name ;
which some understand of Simon Magus, others of antichrist; rather the false Christs are intended, of whom our Lord speaks, in (Matthew 24:24) .

who would rise up of themselves, and not be able to give any proof of their mission; or do anything which might entitle them to the character of the Messiah, or Christ, a name they would take to themselves: and so the Ethiopic version reads, "if another shall come in my name"; saying he is Christ, or the Messiah:

him you will receive ;
as thousands of them did receive Barchocab, the false Christ, who rose up some years after in Adrian's time; and even some of their greatest Rabbins, as particularly the famous R. Akiba, who was his armour bearer:

and it is easy to observe, that though they were so backward to receive, and so much prejudiced against the true Messiah, they were always forward enough to embrace a false one: and indeed to follow any, that set up himself for a temporal deliverer of them; as the instances of Theudas, and Judas of Galilee, with others, show; see (Acts 5:36Acts 5:37) .

And the true reason why they rejected Christ was, because he did not appear in outward pomp and glory, nor set up a temporal kingdom, or give out that he would deliver them from the Roman yoke.