This is what is known as the Great Commission. In the original language, these words are a command. That is why we call this the Great Commission and not the Great Suggestion. And I believe that to fail to do this actually could be a sin.
Sometimes Christians will look at non-Christians as the enemy. But they are not the enemy—they are trapped by the enemy, who is the devil. They are like you and I once were: captives of sin. That is how Christians need to see nonbelievers.