NOTES for Co2 13:5
Paul tried, not once to describe by various words, what is Christian life. And here is now he calls his coreligionists to examine himself: if they are Christian? At first sight, the question may seem strange: because every person entering the Church (and the baptism in those days was accepted only at the age of consciousness), knew perfectly what he does and Who he follows. And all the same, as seen, things are not so simple. Of course, if Christianity was only another new religion, a choice once taken, a promise once pronounced, a baptism once accepted would have been enough: until the one who chose did not abdicate from it, he would remain a Christian.
But indeed, Christianity is not a new religion, but life in the Kingdom, that the Savior brought to the world. It is impossible here to be limited to only a choice, a decision, an action, even if very important. Life does not stop, it continues in the Kingdom, and the choice once taken, must then be confirmed again and again. That is why the apostle advises to examine oneself, to check his spiritual state, to give account, are we living the life of Christ or not: because the life of Christ is the life of the Kingdom, and we remain Christian only as long as we live a common life with Christ. It would seem, for those who are in the Church, it cannot be otherwise.
It would have been in fact like that, if we had already ended the path of our transformation, if our human nature would have been renewed totally, as that will take place in the end of time, when the Kingdom will reveal itself in all its plenitude. Meanwhile, the Kingdom still only enters the world, and we enter the Kingdom, and since the world is not yet transformed to the end, so we also are transformed only partially. In such situation are still possible sins, and errors, and falls.
Prior to the total transformation we still have what to be afraid of. And if that was the case, this constant self-control, of which Paul writes, is necessary. Because every committed sin separates us from the Kingdom, and if committing it we do not lose all the same salvation, it is only because the history of the Kingdom did not end yet, the point is not set, and doors are not closed. But even in such situation it is once again better not to risk and to reduce the possibility of losing the plenitude of the life at least. It is what, in fact the apostle exhorts us.