John Calvin Commentary Psalms 105:42

"For he remembered his holy word, [And] Abraham his servant." — Psalms 105:42 (ASV)

For he remembered his holy promise. The Psalmist again mentions the reason why God dealt so graciously with that people and sustained them so tenderly: namely, that he might fulfill his promise, for he had entered into a covenant with Abraham, committing to be the God of his seed.

Nor did the prophets without reason teach so carefully, as we find them doing, that the free covenant is the fountain from which the deliverance and the continual welfare of the people flowed. In this way, the grace of God became better known, since what took place, so far from happening suddenly and without anticipation, was only the fulfillment of what he had promised four hundred years before.

God then, for ages before this, gave the light of his word of promise, so that his grace and truth might be brought more distinctly into view. For this reason, the prophet again repeats that God was not led by some new cause to deliver his people, but that his design in doing so was to prove the faithfulness of his covenant and to make it effective, just as if a man should dig up from the ground a treasure which he had buried in it.

Nor can it be doubted that the prophet aimed to lead the faith of his countrymen still further—that his object was that their posterity might be persuaded beyond all doubt that, just as God had then proved (in the experience of that generation) the sure and substantial truth of his promise delivered many hundred years before, so he would be to them no different from how their fathers had found him to be in times past.

Accordingly, he designates this promise with the term holy, implying that after the death of Abraham it retained its power and effectiveness undiminished. God had spoken it to Abraham, but the force of the covenant did not die with him. God continued to show himself faithful to the posterity of the patriarch.