Watch and pray, that ye enter not into temptation. The great temptation of the disciples at that moment was to deny Christ under the influence of fear. And so our Lord gives here the true remedy against temptation of every kind; namely, watchfulness and prayer—watchfulness, against the craft and subtlety of the devil or man; and prayer, for the Divine help to overcome.
The spirit indeed is willing, but the flesh is weak. Here our Lord graciously finds excuses for them. It is as though he said, "I know that in heart and mind you are ready to cleave to me, even though the Jews should threaten you with death.
But I know also that your flesh is weak. Pray, then, that the weakness of the flesh may not overcome the strength of the spirit." St. Jerome says, "In whatever degree we trust to the ardor of the spirit, in the same degree ought we to fear because of the infirmity of the flesh."