When a person is reconciled to God by believing in Jesus Christ for forgiveness of sins, he becomes a new creature. But the inherited desire to sin remains. This is concupiscence.
Original sin is a term referring to the Biblical teaching that every human being has inherited the sin of the first man, Adam (Romans 5.12-17). Adam chose to disobey God. In so doing, his nature was completely corrupted. It is this corrupt nature which you inherited from your parents, who got it from theirs, all the way back to Adam, the first created human being.
Concupiscence is evil desire, meaning that we never get over our tendency to be envious, contentious, greedy, self-centered, ill-tempered, and impatient. -Pastor Greg Jackson, Catholic, Lutheran, Protestant, p111
Why the concept matters
One of the marks of a Christianity gone askew is a belief that anyone other than Jesus Christ can live sinlessly and perfectly on this side of eternity. This can only lead to arrogance and a soul-killing faith in one’s own goodness.
As one person with no understanding of the Biblical doctrine of concupiscence writes:
I am an incredibly moral person, given traditional Christian morality. Married faithfully to one woman for 31 years now. Don’t drink, smoke or chew. Give to charities/causes. Work for and with the poor and marginalized. Etc. 1
From a human perspective these proofs of this person’s outward goodness are admirable and even exemplary. But God looks at the heart (1 Samuel 16.7). And the heart of man is deceitful (Jer 17.9) telling us that we are better than we are:
For I say, through the grace given unto me, to every man that is among you, not to think of himself more highly than he ought to think; but to think soberly, according as God hath dealt to every man the measure of faith. (Romans 12.3)
God forgives - and keeps forgiving
Should a born-again Christian keep asking for God’s forgiveness each time she catches herself with a sinful thought or action? Yes
If we say that we have no sin, we deceive ourselves, and the truth is not in us: but, if we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness. (1 John 1.8-9)
I acknowledge my transgressions, and my sin is ever before me. (Psalm 51.9)
Dear Lord, thank you that you are a merciful God, forgiving us seventy-times-seven times. Your goodness is beyond anything we can ask or think. Until that day when we are free of concupiscence, and have new bodies without sin, continue to have mercy. Through Jesus Christ thy Son, our Lord, who lives and reigns with you and the Holy Ghost, one God, world without end, Amen.
- From a public blog post. ^