Mark 14:39

And again, he went away and prayed, saying the same words; and returning again, he found them sleeping. He alone prays for all, just as he alone suffers for all. The eyes of the apostles were weakening and oppressed with the nearby denial.

The threefold sleep of the disciples points out the three dead, whom our Lord raised up; the first, in a house; the second, at the tomb; the third, from the tomb. And the threefold watch of the Lord teaches us in our prayers, to beg for the pardon of past, future, and present sins.

That we may understand what was the cause of His sadness, let us see what precedes and follows this confession of sadness: for in the Passover supper our Lord completely signified the whole mystery of His Passion and our faith. After He had said that they should all be offended in Him, but promised that He would go before them into Galilee, Peter protested that though all the rest should be offended, he would remain faithful and not be offended. But the Lord knowing by His Divine Nature what should come to pass, answered that Peter would deny Him thrice: that we might know from Peter how the others were offended, since even he lapsed into so great peril to his faith by the triple denial. After that, He took Peter, James and John, chosen, the first two to be His martyrs, John to be strengthened for the proclamation of the Gospel, and declared that He was sorrowful unto death. Then He went before, and prayed, saying, "My Father, if it is possible, let this cup pass from Me; yet, not as I will, but as Thou wilt." He prays that the cup may pass from Him, when it was certainly already before Him: for even then was being fulfilled that pouring forth of His blood of the New Testament for the sins of many. He does not pray that it may not be with Him; but that it may pass away from Him. Then He prays that His will may not be done, and wills that what He wishes to be effected, may not be granted Him. For He says, "Yet not as I will, but as Thou wilt:" signifying by His spontaneous prayer for the cup's removal His fellowship with human anxiety, yet associating Himself with the decree of the Will which He shares inseparably with the Father. To shew, moreover, that He does not pray for Himself, and that He seeks only a conditional fulfilment of what He desires and prays for, He prefaces the whole of this request with the words, "My Father, if it is possible." Is there anything for the Father the possibility of which is uncertain? But if nothing is impossible to the Father, we can see on what depends this condition, "if it is possible": for this prayer is immediately followed by the words, "And He came to His disciples and findeth them sleeping, and saith to Peter, Could ye not watch one hour with Me? Watch and pray that ye enter not into temptation: for the spirit indeed is willing, but the flesh is weak." Is the cause of this sadness and this prayer any longer…