Jesus Christ fulfilled the prophecies of the Old Testament - over time, this confirmed he was the Messiah - or, at least, consistent-with being the Messiah.
But this fulfillment only happened over time.
Jesus said he was (the Son of) God, but why believe him during his ministry?
Answer: The miracles. Christ performed miracles - this validated that he had supernatural powers, and therefore his claims were plausible.
(It would have been no sin not to believe in Christ during his life - if it had not been for the miracles. They were enough to inspire belief among true seekers of salvation.)
So, he said he was God, and performed miracles as proof.
But miraculous powers might be good or evil.
Although gifted with miraculous power, Jesus accepted crucifixion without resistance - this implied he was not evil.
Would an evil miracle worker do this?
The Resurrection - (and the Ascension and the coming of the Holy Ghost to the Apostles, and their miracles) confirmed that Christ was God.
Since God was crucified by Man - he revealed the wretchedness of Man, that Man was evil and helpless and needed a Saviour.
(Not completely evil and completely helpless; but hope-less - requiring external help for salvation.)