But. — Rather, for, marking an ellipse easily supplied. Israel’s sense of Jehovah’s readiness to forgive was too deep to need expression, it was understood; “Thou wilt not mark, &c, for...”
Forgiveness. — The article in the original may be more than that common with abstract nouns. “The forgiveness we need.”
That thou mayest be feared. — Either that the forgiven ones may become more profoundly religious, or perhaps, rather, that the manifestation of Divine mercy to Israel may strike fear in the heathen.