For the flesh lusts against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other…
A garrison is not free from danger while it has an enemy lodged within. You may bolt all your doors and fasten all your windows; but if the thieves have placed even a little child within doors, who can draw the bolts for them, the house is still unprotected. All the sea outside a ship cannot do it damage till the water enters within and fills the hold. Hence, it is clear, our greatest danger is from within. All the devils in hell and tempters on earth could do us no injury if there were no corruption in our nature. The sparks will fall harmlessly if there is no tinder. Alas, our heart is our greatest enemy: this is the little home-born thief. Lord, save me from that evil man, myself.
(C. H. Spurgeon.)
Parallel VersesKJV: For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.