Tradition
St. Jerome
Against the Pelagians
Book II › Section 27
27. Who, then, can be saved? It is possible, but to God only.
xxii. 24. The contest for precedence at the last supper.
31, 32. Peter's faith almost overcome by Satan.
Luke xxii. 43. Even Christ in his agony needs an angel to strengthen Him.
Source: Against the Pelagians (New Advent)