Charles H. Spurgeon on Psalms 41:4

God is the strength of a Christian's heart, by healing and restoring him when the infused habits of grace fail, and sin grows strong and vigorous. A Christian never fails in the exercise of grace, but sin gives him a wound; and therefore David prayed, Lord, heal my soul, for I have sinned. And what David prayed for, God promises to his people: "I will heal their backsliding." Ho 14:4. The weakness and decay of grace, brings a Christian presently to the falling sickness; and so it did in David and Ephraim; aye, but God will be a physician to the soul in this case, and will heal their diseases; and so he did David's falling sickness, for which he returned the tribute of praise. Ps 103:3. Samuel Blackerby.