2 John 1:8 KJV
Look to yourselves, that we lose not those things which we have wrought, but that we receive a full reward.
There is an interesting change of person in this verse that
Poole says is, "neither unusual nor... inelegant." The Apostle chooses to warn these beloved brethren that, should they be led astray by false doctrine, “we,” not just “they” would lose the full reward.
Christianity is a community thing. We are not alone as believers but when we rise up we lift up the whole of our Christian body and when we fall we bring down the whole. Cain's sinfulness is manifested not only in the murder of his brother but in his failure to see his responsibility toward Abel; He is his brother's keeper.
Barnes gives good counsel when he writes, "When error abounds in the world, our first duty is not to attack it... (but to) look to the citadel of our own souls, and see that it is well guarded there.”
Perhaps this has been the weakness of much of modern Christianity. It has been on the offense and hasn't prepared well for the defense of those back home. Consequently we have won the souls of strangers (a good work) at the expense of the souls of our loved ones (a tragedy.)