From Charles Spurgeon's The Treasury of David, Psalm 106.
Verse 7. Our fathers understood not thy wonders in Egypt. The Israelites saw the miraculous plagues and ignorantly wondered at them: their design of love, their deep moral and spiritual lessons, and their revelation of the divine power and justice they were unable to perceive. A long sojourn among idolaters had blunted the perceptions of the chosen family, and cruel slavery had ground them down into mental sluggishness. Alas, how many of God's wonders are not understood, or misunderstood by us still.
We fear the sons are no great improvement upon the sires. We inherit from our fathers much sin and little wisdom; they could only leave us what they themselves possessed. We see from this verse that a want of understanding is no excuse for sin, but is itself one count in the indictment against Israel. They remembered not the multitude of thy mercies. The sin of the understanding leads on to the sin of the memory. What is not understood will soon be forgotten. Men feel little interest in preserving husks; if they know nothing of the inner kernel they will take no care of the shells.
It was an aggravation of Israel's sin that when God's mercies were so numerous they yet were able to forget them all. Surely some out of such a multitude of benefits ought to have remained engraven upon their hearts; but if grace does not give us understanding, nature will soon ease out the memory of God's great goodness. But provoked him at the sea, even; at the Red sea. To fall out at starting was a bad sign. Those who did not begin well can hardly be expected to end well. Israel is not quite out of Egypt, and yet she begins to provoke the Lord by doubting his power to deliver, and questioning his faithfulness to his promise. The sea was only called Red, but their sins were scarlet in reality; it was known as the "sea of weeds," but far worse weeds grew in their hearts.