Psalms 116:16

O LORD, truly I am thy servant; I am thy servant, and the son of thine handmaid: thou hast loosed my bonds.

Because the Lord has loosed his bonds, the speaker declares himself the Lord's servant, belonging to Him, acknowledging Him as Lord, and giving willing, grateful, joyful service in return for that freeing love and mercy.
That declaration is the answer to the question of what to give back and what to render to God for His infinite love and mercy.
It is the constant language and answer of one who knows that Jesus loves him, who looks to Christ and follows Him.

That looking to Christ takes away darkness and doubt and gives strength and uplift and the light of life.
Beholding Him changes the beholder into His same image and imparts His same spirit.
It removes all sullenness and gloom, makes experience clear as sunlight, fills the heart with joy and gladness, and makes the life reflect the divine image.