Moses’ actual words were, Yigdal na ko’ach HaShem, “Now let the strength of the Lord be great” (Num. 14:17). Not that God needed to be reminded how powerful He was, but Moses urged Him to show the surrounding nations a sign of His power. Not the power to destroy but the power to save.
Moses qualified his statement about strength and power with these words, “The Lord is slow to anger and generous in lovingkindness, forgiving iniquity and transgression” (Num. 14:18).
Some of the commentators quote the Book of Proverbs, “Whoever is slow to anger is better than the mighty” (Prov. 16:32). This verse later became the basis for Ben Zoma’s saying, “Who is mighty? He who controls his passions” (Avot 4:1).
Does this mean that Moses expects God to act like a human being? We know that Biblical language often attributes human characteristics to God in order to help human beings to understand. But in a sense what may be happening here is that Moses is saying, “Show the nations – including Israel – a sign of the greatness of Your power of forgiveness, and from that we earthly creatures will learn how to try to emulate Your ways”.
After all, since forgiving people is so much harder than wanting to destroy them, we need the Divine example in order to reinforce the capacity to forgive even when forgiving is difficult.