Brian Cosby writes:

While sins of commission are often blatant and deliberate—transgressing a known law or command—sins of omission can be subtle and sneaky. We may not even realize that we have failed to do what God commands. While I might not ever commit adultery, for example, I could easily fail to love my wife as Christ loved the church (Eph. 5:25).

James 4:12 is clear that when we know the right thing to do, but fail to do it, that is sin.

On what occasions are we guilty of sins of omission?

1. when we lack conformity to God’s law in our thoughts and desires—when we do not set our minds on things above (Col. 3:2) or when we fail to love God with all our hearts and minds (Luke 10:27). We are commanded to “take every thought captive to obey Christ” (2 Cor. 10:5) and to “rejoice in the Lord always” (Phil. 4:4). But how often are we doers of these commands?
2. when we lack conformity to God’s law in our words. The Apostle Paul in his letter to the Ephesians exhorts the church to speak the truth in love and to speak “only such as is good for building up” (Eph. 4:15, 29). We should speak truthfully (1 Peter 3:10), giving words of encouragement to one another (1 Thess. 5:11). Our speech should be gracious (Col. 4:6) and gentle (Prov. 15:4). But when we fail to speak in these ways, we commit sins of omission with our speech.
3. when we lack conformity to God’s law in our deeds. The Bible contains a number of positive “one another” commands. For example, we are called to “be at peace with one another” (Mark 9:50), to bear with one another in love (Eph. 4:2), to be “kind to one another, tenderhearted, forgiving one another” (v. 32), and “to seek to do good to one another, and to everyone” (1 Thess. 5:15). The prophet Micah writes, “He has told you, O man, what is good; and what does the Lord require of you but to do justice, and to love kindness, and to walk humbly with your God?” (Mic. 6:8). Jesus teaches that we are to love our enemies and pray for those who persecute us (Matt. 5:44). Then there’s the Ten Commandments, including keeping the Sabbath day holy (Ex. 20:8). When we fail in keeping these positive commands, we commit sins of omission.

We should daily repent of our sins of omission and put our faith in Christ. We should confess—with the saints of old—that “we have left undone those things which we ought to have done” (Book of Common Prayer), neglecting and disregarding God’s positive commands. And when we do, let us rest in the promise of God that He “is faithful and just to forgive us our sins and to cleanse us from all unrighteousness” (1 John 1:9).

(Precis)