Fr. Cornelius a’ Lapide thinks it reasonable to conclude that Christ rode upon both the ass and the colt, though of course not at the same time but in succession. The Jesuit scholar states that it is most likely that the Savior rode the ass down the Mount of Olives and up the hill to the entrance of Jerusalem, but that, when he entered the Holy City, he sat upon the colt.
The primary reason Fr. Cornelius gives for this transitioning between the two beasts is that “the colt perchance was not strong enough to bear a rider in the descent and ascent of the mountain.” The colt, of course, was smaller than the ass (being its foal) and therefore was likely too weak to carry the Christ.
On the other hand, “the ass was not so becoming for the entry into the city.” Therefore, the Lord sat upon the ass’ colt when he came to the City itself. And this is corroborated by the other Gospels which specify that the animal upon which the Savior sat as he came into Jerusalem was a colt.
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