John Calvin Commentary Numbers 14:17

"And now, I pray thee, let the power of the Lord be great, according as thou hast spoken, saying," — Numbers 14:17 (ASV)

And now, I implore you, let the power of my Lord be great. He derives another basis for confidence from the vision, in which God had more clearly revealed His nature. From this vision, it becomes apparent how much he had profited by it and what earnest and anxious attention he had paid to it.

From this, however, we derive a general instruction: there is nothing more effective in our prayers than to present His own word before God and then to base our supplications on His promises, as if He dictated to us from His own mouth what we were to ask.

Since, then, God had revealed Himself to Moses in that memorable declaration, which we have already considered, Moses was able to derive from it a sure guide for prayer. For nothing can be more certain than God's own word.

If our prayers are based on His word, there is no reason to fear that they will be ineffective or that their results will disappoint us, since He who has spoken will prove Himself to be true. And, in fact, this is the reason why He speaks: namely, to provide us with the grounds for addressing Him, for otherwise we would necessarily be silent.