Everyone sins, and has to deal with the effects of sin in their lives. Sin is the rejection of God and His laws and will. It is an act of selfishness, of choosing oneself over God. Since sin is abundant in the world and in each person's life, it is very important to be aware of this reality. If we are cognizant of sin, we can work against it in our own lives, but if we ignore it or deny it, it will continue to dominate us.
Sin is evident in the Bible
Sin is very evident throughout the Bible, as people continually assert their own will against the will of God. “Hear, O heavens, and give ear, O earth; for the Lord has spoken: 'Sons have I reared and brought up, but they have rebelled against me.'” This is very much the story of the Old Testament and even the New Testament and our day. God has given Himself, his grace and his truth to the world and more particularly to individuals, and they continue to rebel against Him and reject Him. Jesus and St. Paul speak of the powerful hold which sin holds in our everyday life. “Jesus answered them, 'Truly truly, I say to you, every one who commits sin is a slave to sin.'” - John 8:34. “Do you not know that if you yield yourselves to any one as obedient slaves, you are slaves of the one whom you obey, either of sin, which leads to death, or of obedience, which leads to righteousness?” - Romans 6:16. These passages remind us that sin is enslaving and that the more we sin, the more we will be inclined to sin in the future. Finally, St. John makes the important distinction between mortal and venial sin. “If any one sees his brother committing what is not a mortal sin, he will ask, and God will give him life for those whose sin is not mortal. There is sin which is mortal; I do not say that one is to pray for that. All wrongdoing is sin, but there is sin which is not mortal.” - 1 John 5: 16 – 17. The apostle makes very clear here the distinction between deadly sin and that which weakens the soul but does not kill it.
Early Church Fathers Sin and its consequences
Sin and its consequences was a very common and important topic to deal with in the writings of the early church fathers, because it is so prevalent and affects everyone in their daily life.
[E]ternal fire was prepared for him who voluntarily departed from God and for all who, without repentance, persevere in apostasy. - Justin Martyr (fragment in Irenaeus, Against Heresies 5:26)
Stinginess is remedied by generosity, insult by apology, perversity by honesty, and for whatever else, amends can be made by practice of the opposite. But what can he do who is contemptuous of God? What shall the murderer do? What remedy shall the fornicator find?...These are capital sins, brethren, these are mortal. Someone may say: ‘Are we then about to perish?...Are we to die in our sins?’...I appeal first to you brethren who refuse penance for your acknowledged crimes. You, I say, who are timid after your impudence, who are bashful after your sins, who are not ashamed to sin but now are ashamed to confess. - Pacian of Barcelona Sermon Exhorting to Penance 4
Types of Sin
There are venial sins and there are mortal sins. It is one thing to owe ten thousand talents, another to owe but a farthing. We shall have to give an accounting for an idle word no less than for adultery. But to be made to blush and to be tortured are not the same thing; not the same thing to grow red in the face and to be in agony for a long time...If we entreat for lesser sins we are granted pardon, but for greater sins, it is difficult to obtain our request. There is a great difference between one sin and another. - Jerome Against Jovinian 2:30
We should fear ourselves, least perchance after [we have come to] the knowledge of Christ, if we do things displeasing to God, we obtain no further forgiveness of sins, but are shut out from His Kingdom. And for that reason, Paul said, ‘For if [God] spared not the natural branches, [take heed] lest He also not spare you” (Romans 11:21). - Irenaeus Against Heresies 4.27.2
These authors take sin very seriously, because they know that committing serious sin can lead to eternal damnation, while venial sin weakens the soul so as to be more inclined to commit great sins. They know that a person must seek forgiveness if they realize that they have sinned.