John Calvin Commentary Numbers 24:9

"He couched, he lay down as a lion, And as a lioness; who shall rouse him up? Blessed be every one that blesseth thee, And cursed be every one that curseth thee." — Numbers 24:9 (ASV)

Blessed is he who blesses you. This manner of expression signifies that the Israelites were elected by God on the condition that He would consider any injury or benefit they might receive as conferred upon Himself.

Nor is there anything new in this, for God to declare that He would be an enemy to the enemies of His Church and, on the other hand, a friend to her friends, which is a sign of the high favor with which He regards her.

From this, however, we are taught that whatever good deeds are performed towards the Church are conferred upon God Himself, who will faithfully reward them; and, at the same time, that believers cannot be injured without His avenging them, just as He says: He who touches you touches the apple of My eye (Zechariah 2:8).

If anyone should object that Balaam himself went unrewarded, although he blessed the people, the reply is simple: he was unworthy of any praise, as he was by no means disposed to favor the people of his own accord and out of pure and generous feeling, but was forcibly drawn in a direction where he was unwilling to go.

Meanwhile, this point remains unshaken: that all who have contributed their labors for the Church’s welfare and have been her faithful helpers will certainly be partakers of the blessing promised here.