To tread on serpents - Preservation from danger. If you tread on a poisonous reptile that would otherwise injure you, I will keep you from danger. If you go among bitter and malignant enemies that would seek your life, I will preserve you. See the notes at Mark 16:18.
Scorpions - The scorpion is an animal with eight feet, eight eyes and a long jointed tail, ending in a pointed weapon or sting. It is found in tropical climates, and seldom exceeds 4 inches in length. Its sting is extremely poisonous, and it is sometimes fatal to life. It is in Scripture the emblem of malicious and crafty men. When rolled up it has some resemblance to an egg, Luke 11:12; Ezekiel 2:6. The annexed cut will give an idea of its usual form and appearance.
The enemy - Satan. The meaning of this verse is, that Jesus would preserve them from the power of Satan and all his emissaries - from all wicked and crafty men; and this shows that he had divine power. He that can control Satan and his hosts that can be present to guard from all their machinations, see all their plans, and destroy all their designs, must be clothed with no less than almighty power.