Genesis 49:17

Therefore, when we are going well on the road, let us beware that a snake does not hide in the path and overthrow the footprint of the horse, that is, of our body, and suddenly cast down the sleeping rider. For if we are watchful, we ought to be somewhat cautious and to avoid the bites of the serpent.

The fact that the prophet says "biting the heel of the horse" signifies that Christ will test those who announce the way of the truth and salvation. So he also tested the apostles, cheated Judas and took hold of him; he took hold of him as of a horse and threw the rider who was on it to death.

The path indicates a more narrow way, which demonstrates that he does not walk along that wide and spacious way leading to death but along that narrow way full of obstacles leading to life. He bites the heel of the horse and so does what the apostle Paul said: "I pommel my body and subdue it, lest after preaching to others I myself should be disqualified." He acts in this manner so that the rider may fall backwards. He always fears elations and heights and prefers his soul to fall from an evil height to an honest humility, in order to learn from Christ, who is modest and gentle in his heart. It is typical of those who progress to fear elation. And finally a very important apostle like Paul said, "A thorn was given me in the flesh, a messenger of Satan, to keep me from being too elated." And so the soul, after being recalled from elation and led back to the humility of Christ, waits for the salvation of the Lord.

(Verses 16 and following.) And he shall judge his people, as one of the tribes of Israel. Let Dan be a serpent by the way, an adder in the path, that biteth the horse heels, so that his rider shall fall backward: thy salvation, O Lord, I will wait for.

In the third mystery, that of the calling of the fathers, the Antichrist is represented by Dan. And some commentators choose to say that he will literally belong to the tribe of Dan. I would not dare to assert that it will be so.