In commenting on Psalm 138.2, Calvin emphasizes the priority of the Word in the communication to us of the divine mercy–God’s “unspeakable mercy” that “condescend[s] to address [us] in a familiar manner.”
I will worship towards the temple of thy holiness. [David] intimates that he would show more than private gratitude, and, in order to set an example before others, come in compliance with the precept of the law into the sanctuary. He worshipped God spiritually, and yet would lift his eyes to those outward symbols which were the means then appointed for drawing the minds of God’s people upwards. He singles out the divine mercy and truth as the subject of his praise, for while the power and greatness of God are equally worthy of commendation, nothing has a more sensible influence in stimulating us to thanksgiving than his free mercy; and in communicating to us of his goodness he opens our mouth to sing his praises. As we cannot taste, or at least have any lively apprehensions in our souls of the divine mercy otherwise than through the word, mention is made of his faithfulness or truth. This coupling of mercy with truth is to be particularly taken notice of, as I have frequently observed, for however much the goodness of God may appear to us in its effects, such is our insensibility that it will never penetrate our minds, unless the word have come to us in the first place. Goodness is first mentioned, because the only ground upon which God shows himself to us as true is his having bound himself by his free promise. And it is in this that his unspeakable mercy shows itself — that he prevents those with it who were at a distance from him, and invites them to draw near to him by condescending to address them in a familiar manner.