John Calvin Commentary Psalms 138:4

"All the kings of the earth shall give thee thanks, O Jehovah, For they have heard the words of thy mouth." — Psalms 138:4 (ASV)

Let all kings of the earth praise thee. Here he declares that the goodness he had experienced would be extensively known, and the report of it spread over all the world.

When he says that even kings had heard the words of God’s mouth, he does not mean to assert that they had been taught the true religion, preparing them to become members of the Church. Rather, he means only that it would be well known everywhere that the reason for his having been preserved in such a wonderful manner was God’s anointing him king by his commandment.

Thus, although the neighboring kings gained no advantage from that divine oracle, the goodness of God was illustrated by its being universally known that he was called to the throne in an extraordinary manner.

Having uniformly declared during the whole period of Saul’s severe and bloody persecution that he raised his standard in God’s name, there could be no doubt that he came to the crown by divine will and commandment.

And this was a proof of divine goodness which might elicit an acknowledgment even from pagan kings.