No one, moreover, so long as he is in this mortal life, ought so far to presume as regards the secret mystery of divine predestination, as to determine for certain that he is assuredly in the number of the predestinate; as if it were true, that he that is justified, either cannot sin any more, or, if he do sin, that he ought to promise himself an assured repentance; for except by special revelation, it cannot be known whom God hath chosen unto Himself.
Why is it false to suppose that one can know for certain whether he is among the predestinate? Because God nowhere promises to give this information to everyone. Consequently only those relative few to whom he freely and graciously makes it known have any basis for confidence about the question. The rest of us live by faith.
Why is this false opinion dangerous? Because a man who comforts himself in his sin that he is among the predestined may delay the day of his repentance, and thereby suffer the loss of salvation if he dies without repenting. It encourages an unwarranted and unbiblical notion of assurance concerning one's salvation - an assurance that Trent condemns, as we have seen.