They went out from us, but they didn't belong to us; for if they had belonged to us, they would have continued with us. But they left, that they might be revealed that none of them belong to us.

 His object in writing is that they may not sin. There is a two-fold connection of thought here--with what has gone before and with what comes afterwards. There is a two-fold danger that they may indeed think lightly of sin. 