Jesus' words to the disciples in Matthew 28:19–20 are often called The Great Commission. And in the original language, these words are a command—not a suggestion.
You might find this statement somewhat controversial, but I believe that to not
preach the gospel actually could be a sin.
Some might say, "Now you are going too far. That is not a sin; maybe it's a shortcoming. Maybe I'm not doing all that I could. But I don't know whether I would classify not sharing my faith as a sin."
But I think it could be a sin. It could be a sin of omission, because the Bible says, "To him who knows to do good and does not do it, to him it is sin" (James 4:17).
Let's say that you were a scientist who came up with a cure for cancer. For some unknown reason, you decided to keep it to yourself and not reveal it to anyone else. Would that be irresponsible? Of course it would. In a way, it would be almost criminal when you consider all the suffering people that could be helped.
Or let's say that you were walking down the street, and there was a house on fire with people inside. You could possibly run in and try to get them out. Or you could at least yell and warn them. At the very least, you could dial 911 so the fire department could rescue them. But you simply say, "Someone else will call. Someone else will take care of it," and you keep walking. Would that be a bad thing? Of course it would.
How much more appalling it is, then, when we know the cure for eternal separation from God, yet we tell no one. If we really believe this, then we must tell others.