Therefore when (when then, οὖν) they were gathered together. The illative particle refers to the fact just mentioned that the notorious Barabbas was at that time in prison. The multitude, together with the Sanhedrists summoned from their meeting in the temple, were gathered at the doors of the Praetorium, when Pilate came out and spoke to them.
Whom will ye that I release unto you? He had great hope that their answer would favour Jesus. When it came to choosing between a vile robber and murderer and a beneficent, moral teacher, common sense would guide the choice aright.
Which is called Christ (Matthew 27:22). In Mark Pilate terms him, "the King of the Jews." He puts before them these two names as the limit of their choice, minor offenders being not worthy of consideration in the lace of these celebrated prisoners.
And he names Christ's claims, as if he would remind the people that in Jesus they possibly had the Messiah whom they desired.