12 Why then did He say, “Mine hour is not yet come?” Rather because, having it in His power when to die, He did not yet see it fit to use that power. Just as we, brethren, say, for example, “Now is the appointed hour for us to go out to celebrate the sacraments.” If we go out before it is necessary, do we not act perversely and absurdly? And because we act only at the proper time, do we therefore in this action regard fate when we so express ourselves? What means then, “Mine hour is not yet come?”
When I know that it is the fitting time for me to suffer, when my suffering will be profitable, then I will willingly suffer. That hour is not yet: that you may preserve both, this, “Mine hour is not yet come;” and that, “I have power to lay down my life, and power to take it again.” He had come, then, having it in His power when to die. And surely it would not have been right were He to die before He had chosen disciples. Had he been a man who had not his hour in his own power, he might have died before he had chosen disciples; and if haply he had died when his disciples were now chosen and instructed, it would be something conferred on him, not his own doing.
But, on the contrary, He who had come having in His power when to go, when to return, how far to advance, and for whom the regions of the grave were open, not only when dying but when rising again; He, I say, in order to show us His Church's hope of immortality, showed in the head what it behooved the members to expect. For He who has risen again in the head will also rise again in all His members. The hour then had not yet come, the fit time was not yet. Disciples had to be called, the kingdom of heaven to be proclaimed, the Lord's divinity to be shown forth in miracles, and His humanity in His very sympathy with mortal men.
For He who hungered because He was man, fed so many thousands with five loaves because He was God; He who slept because He was man, commanded the winds and the waves because He was God. All these things had first to be set forth, that the evangelists might have whereof to write, that there might be what should be preached to the Church. But when He had done as much as He judged to be sufficient, then His hour came, not of necessity, but of will,— not of condition, but of power.
Source: Tractates on the Gospel of John (New Advent)