Thomas Aquinas

“ Therefore, just as when the life of the body is taken away, man's every member and part loses its due disposition, so when the life of justice, which is by faith, is done away, disorder appears in all his members. ”

“ A man may also apostatize from God, by rebelling in his mind against the Divine commandments: and though man may apostatize in both the above ways, he may still remain united to God by faith.
But if he give up the faith, then he seems to turn away from God altogether: and consequently, apostasy simply and absolutely is that whereby a man withdraws from the faith, and is called "apostasy of perfidy." In this way apostasy, simply so called, pertains to unbelief. ”

“ Further, unbelief is an act of the understanding: whereas apostasy seems rather to consist in some outward deed or utterance, or even in some inward act of the will, for it is written (Prov. 6:12-14) : "A man that is an apostate, an unprofitable man walketh with a perverse mouth. He winketh with the eyes, presseth with the foot, speaketh with the finger. With a wicked heart he deviseth evil, and at all times he soweth discord." Moreover if anyone were to have himself circumcised, or to worship at the tomb of Mahomet, he would be deemed an apostate. ”