8 He sometimes also fled from violence Himself, but for the same reason as had led Him to command the apostles to do so: that is, He wanted to fulfil His ministry of teaching; and when it was finished, I do not say He stood firm, but He had no desire even to get from His Father the aid of hosts of angels: finding fault, too, with Peter's sword. He likewise acknowledged, it is true, that His “soul was troubled, even unto death,” and the flesh weak; with the design, (however,) first of all, that by having, as His own, trouble of soul and weakness of the flesh, He might show you that both the substances in Him were truly human; lest, as certain persons have now brought it in, you might be led to think either the flesh or the soul of Christ different from ours; and then, that, by an exhibition of their states, you might be convinced that they have no power at all of themselves without the spirit.
And for this reason He puts first “the willing spirit,” that, looking to the natures respectively of both the substances, you may see that you have in you the spirit's strength as well as the flesh's weakness; and even from this may learn what to do, and by what means to do it, and what to bring under what—the weak, namely, under the strong, that you may not, as is now your fashion, make excuses on the ground of the weakness of the flesh, forsooth, but put out of sight the strength of the spirit.
He also asked of His Father, that if it might be, the cup of suffering should pass from Him. So ask you the like favour; but as He did, holding your position—merely offering supplication, and adding, too, the other words: “but not what I will, but what You will.” But when you run away, how will you make this request? Taking, in that case, into your own hands the removal of the cup from you, and instead of doing what your Father wishes, doing what you wish yourself.
Source: De Fuga in Persecutione (New Advent)