Charles H. Spurgeon on Psalms 91:13

Thou shalt tread upon; thou shalt trample under feet. Thou shalt tread upon them, not accidentally, as a man treads upon an adder or a serpent in the way; but his meaning is, thou shalt intentionally tread upon them like a conqueror, thou shalt tread upon them to testify the dominion over them, so when the Lord Jesus gave that promise (Lu 10:19) to his disciples, that they should do great things, he saith, You shall tread upon serpents; that is, you shall have power to overcome whatsoever may annoy you: serpentine power is all hurtful power, whether literal or mystical. As the Apostle assures all believers (Ro 16:20), "God shall tread down Satan (that old serpent) under your feet shortly." Joseph Caryl.

Verse 13 (second clause). But what is said unto Christ? And thou shalt tread on the lion and dragon. Lion, for overt wrath; dragon for covert lurking. Augustine.