John 13:37

When he had shaken off the dread of being the traitor, and was shown to be one of His own, he afterwards asked boldly himself, while the others held their peace. "What sayest thou, Peter? He said, 'thou canst not,' and thou sayest, 'I can'? Therefore thou shalt know from this temptation that thy love is nothing without the presence of the impulse from above." Whence it is clear that in care for him He allowed even that fall. He desired indeed to teach him even by the first words, but when he continued in his vehemence, He did not indeed throw or force him into the denial, but left him alone, that he might learn his own weakness.

But what does he say to this? "Why cannot I follow Thee now? I will lay down my life for Thy sake." He saw what was the kind of desire in his mind; but what the measure of his strength, he saw not. The weak man boasted of his willingness, but the Physician had an eye on the state of his health; the one promised, the Other foreknew: the ignorant was bold; He that foreknew all, condescended to teach. How much had Peter taken upon himself, by looking only at what he wished, and having no knowledge of what he was able! How much had he taken upon himself, that, when the Lord had come to lay down His life for His friends, and so for him also, he should have the assurance to offer to do the same for the Lord; and while as yet Christ's life was not laid down for himself, he should promise to lay down his own life for Christ!

A great thing is love, and stronger than fire itself, and it goeth up to the very heaven; there is no hindrance which can restrain its tearing force. And so the most fervent Peter, when he hears, "Whither I go ye cannot come," what saith he? "Lord, whither goest thou?" and this he said, not so much from wish to learn, as from desire to follow. To say openly, "I go," he dared not yet, but, "Whither goest thou?" Christ answered, not to his words, but to his thoughts. For that this was his wish, is clear from what Christ said, "Whither I go thou canst not follow Me now." Seest thou that he longed for the following Him, and therefore asked the question? And when he heard, "thou shalt follow Me afterwards" not even so did he restrain his longing, and, though he had gained good hopes, he is so eager as to say, "Why cannot I follow Thee now? I will lay down my life for Thee."