That He will do so comes from His very nature. With Him there is no
change of disposition, no emergence of unseen circumstances, no failure
or exhaustion of power.
That He does so is matter of fact. Moses in the preceding context had
pointed to facts of history, on which he built the 'know therefore' of
the text. On the broad scale the whole world's history is full of
illustrations of God's faithfulness to His promises and His threats.
The history of Judaism, the sorrows of nations, and the complications
of national events, all illustrate this fact.
The personal history of each of us. The experience of all Christian
souls. No man ever trusted in Him and was ashamed. He wills that we
should put Him to the proof.