The first, sins of commission, is a category of sin describing the things we did and shouldn't have. I committed (commission) a sin when I lied and I shouldn't have.
The second, sins of omission, are the category that encompasses the sins of not doing what we should have. We don't think as much about the sin of omission even though it is as pernicious and destructive as anything we could commit. I should have testified because I knew he was innocent, but I didn't out of fear. When we know the right thing to do and don't, that's a sin.
Solomon said, "Deliver those who are being taken away to death, And those who are staggering to slaughter, Oh hold them back" Pro. 24:11. Sin is not just something we do, but sometimes it is something that we ought to have done.
Often we do what God has forbidden. We are very conscious of these things because they are visible. Of this kind, Paul says,
"For he who does wrong will receive the consequences of the wrong which he has done, and that without partiality." - Col 3:25We know what Paul is talking about. We often repent of doing these things. But what about the other kind of sin, the sin of omission? Sometimes we sit on our hands and say or do nothing when we should have acted. James says,
"Therefore, to one who knows the right thing to do and does not do it, to him it is sin." - James 4:17One Commentary describes it as the sin of "knowledge without practice" (here). This person's "faith" is empty if he only offers well-wishes to the cold and hungry person. When you know the right thing to do and have the means, then you offer the coat or food; to do nothing is a sin. Paul was guilty of it, saying, "the good that I want to do I do not do" (Rom. 7:16). We often think of sin as the things we should not have done. However being a Christian is characterized as doing what we are positively called to do. A person can't say, "I decided I would not sin today and so I didn't." "How did you do that?" "I just sat down so that I would not step into sin, and I thought about nothing so that I would not think evil." "I see, but don't you think you committed the sin of being the useless fig tree?" "I didn't think of that." The command by Jesus to "deny yourself, and take up your cross and follow me" encompass the things we ought not to do, but it also includes what we ought to. And so the Christian quits drinking and cursing AND he praises God and tells the Good News to others (Mt. 28:18-20; Acts 2:38-41). To not do the latter things is sin.
In a recent lesson we learned how God commanded Joshua and Zerubbabel, the High Priest and the Governor of the remnant of Israel, to "walk in My ways" and "perform My service". If they would, God said, "you will also govern My house and also have charge of My courts, and I will grant you free access among these who are standing here" (Zechariah 3:17). The house of the Lord needed to be built. The service of God needed to be conducted. The idea that someone can be pleasing to God by turning from evil while remaining idle so as not to advance the work is the sin of omission. Truthfully, we understand that we must turn away from Satan's vices. But Satan is just as happy if we sit idly by and let others do the work.
In closing, let each person consider the personal application of Christ's words to: "Do unto others, as you would have them do unto you", and that, "It is more blessed to give, than to receive", and finally, "Go into all the world and preach the Gospel", Jesus said. By doing God's will, we avoid the sin of omission.