Knowledge is hard for us to get. As Aristotle says:
The investigation of the truth is in one way hard, in another easy. An indication of this is found in the fact that no one is able to attain the truth adequately, while, on the other hand, we do not collectively fail, but every one says something true about the nature of things, and while individually we contribute little or nothing to the truth, by the union of all a considerable amount is amassed. Therefore, since the truth seems to be like the proverbial door, which no one can fail to hit, in this respect it must be easy, but the fact that we can have a whole truth and not the particular part we aim at shows the difficulty of it. [Metaphysics II.1 (993b1-11)]
On top of this, we face another problem while seeking knowledge, and that is that our gifts for acquiring knowledge are limited, and different people have different gifts for doing so: some folks are better at it than others.
Consider the case of two persons of whom one has a more penetrating grasp of a thing by his intellect than, does the other. He who has the superior intellect understands many things that the other cannot grasp at all. [Summa Contra Gentiles, I, iii, 4]
If humility is warranted about normal everyday things that we can see and hear and touch, a fortiori we ought to avoid overconfidence with regard to the things of God, whom we cannot see:
We do not know a great many of the properties of sensible things, and in most cases we are not able to discover fully the natures of those properties that we apprehend by the sense. Much more is it the case, therefore, that the human reason is not equal to the task of investigating all the intelligible characteristics of that most excellent substance [i.e., God] (ibid., I, iii, 5).
This is not to say that there is no means by which we may obtain certainty about divine things, for God has revealed Himself to us and has told us many things about Himself and the world, and He has given us the Church to proclaim those truths to us.
With these things in view, it seems wise to retain a prudent humility concerning our own grasp of the Truth. We may indeed get things right, and we should indeed hold firmly to the truth, but we’re limited, fallible, and error-prone, so an obstinate refusal to consider the possibility that we might be wrong is a mistake.