Christ’s prayer gives certainty to us, tremendous certainty. ‘While I was with them in the world, I kept them in thy name: those that thou gavest me I have kept, and none of them is lost,’ with the exception of the son of perdition, who was never saved.
Every Christian may fall, every Christian may sadly have a time of backsliding. But supposing there is a Christian who is always backsliding, hardly ever up but always down, like a moth to the light, drawn to worldly things. He does not fall into sin once, and then deeply regret it, and repent of the sin, and strive to have the blessing of God restored. He falls and you pull him up, and the next day down he goes again, and again. What is the explanation? He cannot be kept because he has never been saved, here is the proof. Because yes, we may have sad falls but if we are truly saved, the evidence of our being kept is there. We are disciplined perhaps by the Lord, we are restored and we struggle to serve him. You may go down again, but it will not be an habitual thing if you are truly saved because you are kept by the Lord from true disasters and great falls.