13 These three evangelists, therefore, have in this manner narrated the same incident, just as, on the other hand, one man might give three several accounts of a single occurrence, with a certain measure of diversity in his statements, and yet without any real contradiction. Luke, for example, has specified the distance to which He went forward from the disciples— that is to say, when He withdrew from them in order to pray— more definitely than the others. For he tells us that it was “about a stone's cast.”
Mark, again, states first of all in his own words how the Lord prayed that, “If it were possible, the hour might pass from Him,” referring to the hour of His Passion, which he also expresses presently by the term “cup.” He then reproduces the Lord's own words, in the following manner: “Abba, Father, all things are possible to You: take away this cup from me.” And if we connect with these terms the clause which is given by the other two evangelists, and for which Mark himself has also already introduced a clear parallel, presented as a statement made in his own person instead of the Lord's, the whole sentence will be exhibited in this form: “Father, if it be possible, (for) all things are possible unto You, take away this cup from me.”
And it will be so put just to prevent any one from supposing that He made the Father's power less than it is when He said, “If it be possible.” For thus His words were not, “If You can do it;” but “If it be possible.” And anything is possible which He wills. Therefore, the expression, “If it be possible,” has here just the same force as, “If You will.” For Mark has made the sense in which the phrase, “If it be possible,” is to be taken quite plain, when he says, “All things are possible unto You.”
And further, the fact that these writers have recorded how He said, “Nevertheless, not what I will, but what You will” (an expression which means precisely the same as this other form, “Nevertheless, not my will but Yours be done”), shows us clearly enough that it was with reference not to any absolute impossibility on the Father's side, but only to His will, that these words, “If it be possible,” were spoken. This is made the more apparent by the plainer statement which Luke has presented to the same effect.
For his version is not, “If it be possible,” but, “If You be willing.” And to this clearer declaration of what was really meant we may add, with the effect of still greater clearness, the clause which Mark has inserted, so that the whole will proceed thus: “If You be willing, (for) all things are possible unto You, take away this cup from me.”
Source: The Harmony of the Gospels (New Advent)