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plt said:
One of the several reasons that Jews reject Jesus as the Messiah is that it is claimed that He is without sin, yet he was baptised. Could someone explain why that was necessary?

What would have happened if Jesus had not been baptised? People could have thought three erroneous things. One, that Jesus proudly thought himself better than others. Two, that Jesus thought that baptism for repentance was unnecessary for anyone. Three, that Jesus for some reason disapproved of John.

We must remember that, at the time, few people knew who Jesus was. We must remember that he did not even want people to recognise him on account of his own claim. He referred to himself as 'the son of man' (which we, with hindsight, write as 'the Son of Man'). It was after he had decided to be baptised, after he had been willing to be seen to be the sinner he was not, that there was glory and recognition given to him by supernal means.