"On the other hand, you must realise from the outset that the goal toward which He is beginning to guide you is absolute perfection; and no power in the whole universe, except you yourself, can prevent Him from taking you to that goal."

Perhaps Lewis had in mind Paul's words in 1 Thessalonians 5:23-24 here. "And the very God of peace sanctify you wholly; and I pray God your whole spirit and soul and body be preserved blameless unto the coming of our Lord Jesus Christ. Faithful is he that calleth you, who also will do it."