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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.
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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.
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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?
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The Resurrection - (and the Ascension and the coming of the Holy Ghost to the Apostles, and their miracles) confirmed that Christ was God.
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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.)
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