In the book Letters Along the Way, DA Carson offers sage advice to Christians on a variety of topics.  This excerpt below covers topics related to sin and guilt.

Guilt after becoming a Christian
On becoming a Christian, your view of right and wrong changed.  Prayerlessness would not have made you feel guilty before; now it does.  Resentment…never troubled you before…now you are appalled that such self-serving behavior is so deeply rooted in your personality.  Before prolonged pandering to a secret lust never struck you as evil – nor did barracks-room jokes or overt flirtation.  Now you find you are far more chained to lust than you could have imagined.  Worst of all, you are finding how impossibly difficult it is …to love God with all our heart and soul and mind and strength, and to love our neighbor as ourselves.

In one sense, this feeling that you are awash in guilt is a good sign.  It means that you are taking sin seriously, and that is one of the marks of a true believer.  John Owen said, “He that has slight thoughts of sin never had great thoughts of God.”

Degrees of sin and resulting punishment
Under the Old Testament law…there were different punishments for different sins.  Jesus insists that on the last day, some will be beaten with more stripes and some with fewer.  In Matthew 11:20-24 Jesus warns the cities of Galilee that had heard Him preach and had witnessed His miracles that their plight on the day of judgment would be much more severe than that of Sodom and Gomorrah, proverbial for wickedness, or of Tyre and Sidon, pagan cities on the coast.  Jesus argument is not that the cities of Galilee were indulging in practices that a detached human, observer would have classified as more vile than those of the pagan cities, but that their privileges were so great – having not only been taught the Scriptures, but having also observed Jesus and listened to His words – that their failure to repent represented a deeper moral failure than socially “worse” sins.  In other words, God takes into account our heritage, our background, our advantages when He judges us….the Bible does not treat all sins as exactly the same.

Persistence of sin
In John 1, John says that on one hand, if anyone claims he does not sin or has not sinned, he is a liar, self-deceived, guilty of calling God a liar.  On the other hand, John insists that Christians do not go on sinning, that they obey Christ and love brothers.  How can both emphases be true?

In fact, unless you hold both emphases strongly and simultaneously, you will go seriously astray.  Stress the former, and you will become lackadaisical about sin; stress the latter, and you may gravitate toward some version of Christian perfectionism where you hod you have already attained perfection…The fact is that until Jesus’ return, we will sin.  As we grow in holiness, we will become aware of inconsistencies and taints we had not seven spotted before.  Most of us will sometimes stumble and drift, at times rather seriously.  There will be different rates of progress, different degrees of spiritual maturity; all of us will have to return to Jesus for renewed cleansing and forgiveness.  But at the same time, if we are Christians, we will insist that there is never any excuse for sin.  In no case do we have to sin….Sinning is simply not allowed in the Christian way.  No provision must be permitted to encourage it; no excuse ever justifies it.

We live in this tension.  The only solution is not a theoretical one, but a practical one, an existential one.  “If we claim to be without sin, we deceive ourselves and the truth is not in us.  If we confess our sins, God is faithful and just and will forgive us our sins and purify us from all unrighteousness” (1 John 1:8-9).

Never, treat God’s forgiveness lightly, as if you may sin with impunity because God is there to forgive you; but never, never wallow in guilt of some sin you have committed in the fear that God is not merciful enough or gracious enough to forgive you.  Learn not to flirt with sin; and when you fall, learn to beg God’s forgiveness for Jesus’ sake and press on.  That is the only way you can live with a clean conscience; it is the only way that your confession of Jesus as Lord will have any bite in your life.