Whence this custom came is uncertain; most probably from the Romans, who in some honour of this great festival of the Jews, and in humour of them, granted them the life of any criminal whom they desired. Pilate propounds Christ as the prisoner whom he had most mind to release, perceiving that his prosecution was of malice, rather than for any just cause.
Related Commentaries of John 18:39
John 18:39
39 But ye have a custom, that I should release unto you one at the passover: will ye therefore that I release unto you the King of the Jews?