From Evangelism and the Sovereignty of God by J. I. Packer
The Gospel is a Message About Sin. It tells us how we have fallen short of God's standard; how we have become guilty, filthy, and helpless in sin, and now stand under the wrath of God. It tells us that the reason we sin continually is that we are sinners by nature, and that nothing we do, or try to do, for ourselves can put us right or bring us back into God's favor....Thus it leads us to self-despair. And this is also a necessary step. Not until we have learned our need to get right with God, and our inability to do so by any effort of our own, can we come to know the Christ, who saves from sin.
There is a pitfall here. Everybody's life includes things which cause dissatisfaction and shame. Everyone has had a bad conscience about some things in the past, matters in which he has fallen short of the standard which he set for himself, or which was expected by others.
The danger is that in our evangelism we should content ourselves with evoking thoughts of these things and making people feel uncomfortable about them, and then depicting Christ as the One who saves us from these elements in ourselves, without even raising the question of our relationship with God.
But this is just the question that has to be raised when we talk about sin. For the very idea of sin in the Bible is of an offense against God, which disrupts man's relationship with God.
Unless we see our shortcomings in the light of the law and holiness of God, we do not see them as sin at all. For sin is not a social concept; it is a theological concept. Though sin is committed by man, and many sins are committed against society, sin cannot be defined in terms of either man or society. We never learn what sin really is until we have learned to think of it in terms of God, and to measure it, not by human standards, but by the yardstick of His total demand on our lives.
What we have to grasp then, is that the bad conscience of the natural man is not all the same thing as conviction of sin. It does not follow then, that a man is convicted of sin when he is distressed about his weaknesses and the wrong things he has done.
It is not conviction of sin just to feel miserable about yourself and your failures, or would it be saving faith if a man in that condition called on the Lord Jesus Christ just to soothe him, cheer him up and make him feel confident again.
Nor would we be preaching the gospel (though we might imagine we are) if all we did was to present Christ in terms of a human's felt-wants ("Are you happy? Are you satisfied? Do you want peace of mind? Do you feel you have failed? Are you fed up with yourself? Do you want a friend? Then come to Christ--He will meet your every need--as if the Lord Jesus Christ were to be thought of as a fairy godmother or a super-psychiatrist).
No; we have to go deeper than that. To preach sin means not to make capital out of people's felt frailties (the brain washer's trick), but to measure their lives by the holy law of God. To be convicted of sin means not just to feel that one is an all-around flop, but to realize that one has offended God, flouted His authority, defied Him, gone against Him, and put oneself in the wrong with Him.
To preach Christ means to set Him forth as the One who, through His cross, sets man right with God again. To put faith in Christ means relying on Him, and Him alone, to restore us to God's fellowship and favor.
[How do we know we are doing it right?]
Conviction of sin is essentially an awareness of a wrong relationship with God.
It is not just a wrong relationship with one's neighbor, or one's own conscience, but with one's Maker, the God in whose hand one's breath is and on whom one depends for existence every moment.
To be convicted of sin is to be aware of one's need of what Ezekiel called a "new heart" (Ezekiel 36:26) and of a "new birth" as Christ told Nicodemus in John 3:16.
J. I Packer goes on to say:
Perhaps the shortest way to determine if a person is convicted of sin or not, is to go to Psalm 51.
V. 1. Have mercy on me, O God,
according to your unfailing love;
according to your great compassion blot out my transgressions.
V. 2-3. Wash away all my iniquity and cleanse me from my sin.
For I know my transgressions,And my sin is always before me.
V. 4. Against you, you only, have I sinned
And done what is evil in your sight, so that you are proved right when you speak and justified when you judge.
V. 5. Surely I was sinful at birth,
Sinful from the time my mother conceived me.V. 6. Surely you desire truth in the inner parts;
You teach me wisdom in
the inmost place.
V. 7. Cleanse me with hyssop,
and I will be clean.
Wash me, and I will be whiter that snow.V. 8. Let me hear joy and gladness.
Let the bones you have crushed
rejoice.
V. 9. Hide your face from my sins
And blot out all my iniquity.
V. 10. Create in me a pure heart, O God,
And renew a steadfast spirit within me.
-- Psalm 51: 1-10
In this psalm David is confessing not only his specific sin (with Bathsheba and Uriah) in verses 1-4, but also the depravity of his nature in verses 5-6 and seeks cleansing from both in verses 7-10.
Probably the best way to find out if a person is convicted of his sin or not is to take him right to this psalm and see if his heart is speaking anything like the psalmist is in these words.
If we confess our sins, He is faithful and just
to forgive us our sins and purify us from all
unrighteousness.
I John 1:9