For I was envious at the foolish - The word “foolish” here refers to sinners. It may either refer to them as foolish, or as proud, insolent, vain - for so the word is elsewhere used. See Psalms 14:1.
When I saw the prosperity of the wicked - More literally, “the peace of the wicked.” The reference is not so much to their prosperity in general as to their peace; their conscious safety; their freedom from trouble; and especially their calmness, and their freedom from suffering, in death. From all this he was led for the moment to doubt whether there was any advantage in religion; whether God was just; and whether he befriended the righteous anymore than he did the wicked.