What he does not mean simply is the Protestant notion of faith as trust – although of course one must trust God (it must be said that if one knows God by grace in the way that St. Thomas says in ST I-II Q113 A4, then it's absurd to suggest that one could do other than trust him: for if you do not trust him, then by the very nature of the case you do not have that knowledge of him that comes by grace; so that while trust is necessary, it seems also to be an inescapable concomitant of the knowledge of which Thomas speaks) – is assent.
That this is so seems clear, for example, from his usage of Hebrews 11:6:
But without faith it is impossible to please God. For he that cometh to God must believe that he is: and is a rewarder to them that seek him.
Believing that God is does not amount to an act of trust, which would be absurd. Clearly it must be an act of assent – in other words, to believe in fact that God really does exist: not by way of trusting someone's word about it, but rather by assenting to the fact that he is.
(Of course, as Hebrews says we must also believe that he rewards those that seek him. Reward implies merit, but Protestants say there is no sense in which God can be said to reward us, which makes this a difficult passage for them to say the least. Meanwhile we as Catholics know that we can affirm what the passage says, because we know that God does reward the merits that he gives to us. But I digress.)
Faith in the Catholic sense – in the sense intended by St. Thomas – implies assent to the dogmas of the faith. This faith need not be explicit, as we have also seen, but it cannot be absent.