Out of the depths One cries for mercy and the crier hopes in God’s Word of promise because he knows the essential character of God: with the Lord there is forgiveness, mercy, and redemption, because that’s who God is and that’s what God likes to give. Therefore, one can hope in God in the dark and dangerous night just as surely as he knows the sun will come up in the morning.
“Depths” means coming close to death which means sensing separation from God, which is caused by Sin. The despairer turns to God and away from sin for he desperately needs to be “with Him.” Forgiveness, mercy and redemption turns away Sin and Death. The NT believer finds God at the Cross.
The experience of knowing I am with that kind of God causes me to know that He may be feared, meaning I now strongly desire to obey Him. Shakespeare was wrong when he wrote,”Nothing emboldens sin so much as mercy.” Truth is the opposite: substitute “obedience” for “sin” in that quote. In fact, mercy is the only means to produce change for the better; the Gospel is the only power to effect proper Fear of God.