Charles H. Spurgeon on Psalms 106:12

Then believed they his words. That is to say, they believed the promise when they saw it fulfilled, but not till then. This is mentioned, not to their credit, but to their shame. Those who do not believe the Lord's word till they see it performed are not believers at all. Who would not believe when the fact stares them in the face? The Egyptians would have done as much as this. They sang his praise. How could they do otherwise? Their song was very excellent, and is the type of the song of heaven; but sweet as it was, it was quite as short, and when it was ended they fell to murmuring. "They sang his praise, "but "they soon forgat his works." Between Israel singing and Israel sinning there was scarce a step. Their song was good while it lasted, but it was no sooner begun than over.