He continues in this strain: “If he had determined upon these things, and underwent chastisement in obedience to his Father, it is manifest that, being a God, and submitting voluntarily, those things that were done agreeably to his own decision were neither painful nor distressing.” But he did not observe that here he was at once contradicting himself. For if he granted that He was chastised because He had determined upon these things, and had submitted Himself to His Father, it is clear that He actually suffered punishment, and it was impossible that what was inflicted on Him by His chastisers should not be painful, because pain is an involuntary thing.
But if, because He was willing to suffer, His inflictions were neither painful nor distressing, how did He grant that “He was chastised?” He did not perceive that when Jesus had once, by His birth, assumed a body, He assumed one which was capable both of suffering pains, and those distresses incidental to humanity, if we are to understand by distresses what no one voluntarily chooses. Since, therefore, He voluntarily assumed a body, not wholly of a different nature from that of human flesh, so along with His body He assumed also its sufferings and distresses, which it was not in His power to avoid enduring, it being in the power of those who inflicted them to send upon Him things distressing and painful.
And in the preceding pages we have already shown, that He would not have come into the hands of men had He not so willed. But He did come, because He was willing to come, and because it was manifest beforehand that His dying upon behalf of men would be of advantage to the whole human race.
Source: Against Celsus (New Advent)