Don't you worry, Stryder---I heard that on MacArthur this morning too. I had a couple reactions. The first reaction is that earlier in the sermon, he talked about how some people have false assurance even though they think they have true assurance. Despite the reality that someone can genuinely be deceived into thinking one is elect, he proceeds later in the episode to say assurance of salvation is some kind of metaphysical absolute certitude. And yet there are some people who can think this and be wrong....? He did not make sense and contradicted himself within 15 minutes.
Secondly, what MacArthur partially quoted from Trent was 6th session, paragraph 12, which I will reproduce here:
CHAPTER XII.
That a rash presumptuousness in the matter of Predestination is to be avoided. 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.
What this paragraph says is to avoid the sin of presumption, which MacArthur condemned as "false assuredness" earlier in the very same sermon! The paragraph says that God is the judge of souls, not us. We can have a moral certainty that we are in God's good graces, but Trent warns us against thinking our imperfect minds have made an absolutely correct conclusion in thinking we are saved such that we "cannot sin any more" or will absolutely surely "repent" some day of our sins.
I've heard him in other sermons try to take the Catholic caution against presumption the way Luther did---that one must live scrupulously to be Catholic, thinking one will go to hell unless he constantly exercise "works righteousness." MacArthur takes caution against presumption as if a Catholic has no capacity to sense he is following God at all, instead of exercising trust and follow God as best he can without being scrupulous. In this regard, MacArthur totally fails to understand and subsequently misrepresents Catholic theology.
At any rate, MacArthur's "absolute assurance" position is destroyed by Biblical concepts like "work out your salvation with fear and trembling" or Paul fearing that he may be "disqualified from attaining the prize."