Romans 14:4
Who are you to pass judgment on the servant of another? It is before his own master that he stands or falls. And he will be upheld, for the Lord is able to make him stand.
Who exactly is Joel serving?
God.
Does he agree that Jesus came in the flesh, died for propitiation of man's sin, and rose into life eternal?
(1Jn 3:23-4:3)You see the Resurrection is proof that Jesus is who He said He was/is/will-be: without Him coming in the flesh he didn't really die, and if he didn't really die then the resurrection is a lie, and that would make Him a liar and Christianity false.
And this is his commandment, that we should believe in the name of his Son Jesus Christ and love one another, just as he has commanded us. All who obey his commandments abide in him, and he abides in them. And by this we know that he abides in us, by the Spirit that he has given us.
Beloved, do not believe every spirit, but test the spirits to see whether they are from God; for many false prophets have gone out into the world. By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God, and every spirit that does not confess Jesus is not from God. And this is the spirit of the antichrist, of which you have heard that it is coming; and now it is already in the world.