See the notes at Matthew 27:15.

For of necessity he must release one - That is, he was under the necessity of releasing one at this feast. The custom, however it originated, had now been so completely established that Pilate was obliged to attend to it. See on Matthew 27:15 (note).

For of necessity he must release one,.... And therefore proposed to release Jesus

unto them at the feast; of passover, which now was; not that there was any law that obliged to it, but it having been customary with the Roman governor to do so, the people expected it; custom had made it necessary: and so the Syriac, Arabic, and Persic versions render it, "it was a custom to release", &c. not at each feast, or every feast, as the last of these versions read, only at the passover, as is expressed, John 18:39.

(For of necessity he must release one unto them at the feast.)

23:13-25 The fear of man brings many into this snare, that they will do an unjust thing, against their consciences, rather than get into trouble. Pilate declares Jesus innocent, and has a mind to release him; yet, to please the people, he would punish him as an evil-doer. If no fault be found in him, why chastise him? Pilate yielded at length; he had not courage to go against so strong a stream. He delivered Jesus to their will, to be crucified.