“He is like a tree planted by streams of water that yields its fruit in its season, and its leaf does not wither. In all that he does, he prospers.” (Psalm 1:3)
“The psalmist here uses a metaphor to confirm the statement he made in the preceding verse. He shows us how those who fear God are to be accounted happy, not because of an evanescent and empty gladness, but because of their desirable condition. These words imply a contrast between the vigor of a tree planted in a well-watered area, and the condition of one that flourishes beautifully for a time, yet soon withers because of the barrenness of the soil in which it is placed.
The ungodly are sometimes like the cedars of Lebanon (Ps. 37:35). They have such an overflowing abundance of wealth and honor that they seem to lack nothing. But however high they may rise, and however far and wide they may spread their branches, they are poorly rooted in the ground and lack enough moisture to derive nourishment. So eventually their beauty withers away. Only the blessing of God can preserve and prosper those whom he plants.
God also allows the godly to bring forth fruit in season, the psalmist says, meaning that the children of God will flourish by being watered with the secret influences of divine grace, so that whatever befalls them is conducive to their salvation. The ungodly, on the other hand, will be carried away by a sudden storm or consumed by scorching heat. The godly will also bring forth fruit in their season, meaning that the fruit they produce will be fully mature. The ungodly, by contrast, may appear to be producing fruit but in the end will produce nothing that comes to perfection.”