David
Psalm 40:4BSB·traditional attribution

Blessed is the man who has made the LORD his trust, who has not turned to the proud, nor to those who lapse into falsehood.

John Calvin Reformed @genevareformer

PSALM 40 David, being delivered from some great danger, and it may be, not from one only, but from many, extols very highly the grace of God, and by means of this, his soul is filled with admiration of the providence of God, which extends itself to the whole human race.

Commenting on Psalm 40:1-17

C.H. Spurgeon Reformed Baptist @princeofpreachers

Blessed. This is an exclamation similar to that of the first Psalm, "Oh, the happiness of the man." God's blessings are emphatic, "I wot that he whom thou blessest is blessed, "indeed and in very truth. Is that man that maketh the Lord his trust. Faith obtaineth promises. A simple single eyed confidence in God is the sure mark of blessedness.

Matthew Henry Presbyterian @wholebiblehenry

In these verses we have, I. The great distress and trouble that the psalmist had been in. He had been plunged into a horrible pit and into miry clay (Psa 40:2), out of which he could not work himself, and in which he found himself sinking yet further.

Commenting on Psalm 40:1-5