Even after conversion he carries within him a nature prone to evil, and a heart weak and unstable as water. That heart will never be free from imperfection in this world, and it is a miserable delusion to expect it. To keep that heart from going astray, the Lord Jesus bids us “watch and pray.” The spirit may be ready, but the flesh is weak. There is need of a daily struggle and a daily wrestling in prayer (Mk. 14:38; 1 Cor. 9:27; Rom. 7:23-24; Gal. 5:24; Col. 3:5).
J.C. Ryle