Why do believers still sin even after reckoning the old man dead?
Believers still sin because the old Adam nature remains powerful. Even though it is reckoned dead according to Romans, it rises up, gets its victories, and causes sin. God, being omniscient, knows believers will sin, though He never encourages sin; therefore John writes that if any man sin, believers have an Advocate.
Les explains that John is writing to believers, not to the unsaved world. He says, “My little children, these things I write unto you that you sin not.” God never encourages the believer to sin, but being omniscient and the Creator, He knows we are going to sin; some may be quicker than others, but we will sin. The old Adam is so powerful that even though we reckon him as dead according to Romans, yet he pops up, gets his licks in, and we sin. Then, because God knows this, John says if any man sin, we have an Advocate with the Father, Jesus Christ the righteous One. When a believer sins, the Advocate immediately comes to the Father and says, “Father, he’s under the blood; she’s under the blood,” and He picks it up on our behalf.