In Article 1, St. Thomas argued that the formal object of faith is the first Truth, God; then in Article 2, he balanced this point against the fact that the object of faith is received in the mode of the receiver, that is to say, that faith forms language and arguments about the simple and unchangeable object of faith. Here in Article 3, St. Thomas continues his train of thought by asking whether faith can assent to something false. His answer, put simply, is that faith cannot assent to something false insofar as its proper object is the first Truth, but that falsehood can kind of creep in on the sides due to the human mode of receiving the object.
The Respondeo is fairly straightforward: strictly speaking, the act of faith is to believe something revealed by Truth itself, and so the assent of faith can never be assent to a falsehood. This would be like getting badness from Goodness, or non-being from Being.
But the objections and replies are subtle and interesting. Objection 3 asks about Jews who believed that the Christ was to be born at some future time: shortly after Christ’s birth, lots and lots of good and faithful Jews believed that the Christ was not yet born, which was false. And yet it would seem that their belief was a consequence of faith! Objection 4 brings the problem closer to home for a Catholic today: we believe on faith that the body of Christ lies under the appearances of bread in the Eucharist; however, a priest could go through the motions of the Mass without intending to consecrate the host, and then the body of Christ would not be present and yet we would still believe—seemingly on faith!
What is interesting about these objections is that they are examples of necessary error. Think about what it would be like, as a Catholic, to wonder at every Mass whether the priest actually intended to consecrate the host; to withhold judgment; to receive communion in doubt, or perhaps to refrain from communion due to perpetual doubt. It would not be sustainable as a Catholic life.
Or think about what it would have been like as a Jew over the centuries after the prophets spoke to get up every morning wondering whether, somewhere out there, the Christ was already born and living a secret life. Even if a thoughtful Jew entertained the possibility, in practicality he would get up and prepare for another day, another week, another year of living before the Christ. Humanly speaking, it was necessary that the Jews go on assuming that Christ had not come until word of him reached them.
St. Thomas’s response to the objection about the Jews is that these good and faithful people combined something of faith (“The Christ is to be born at some time”) with something of their own conjecture (“That time has not yet come”). And similarly the Catholic venerating an unconsecrated host is combining something of faith (“The body of Christ lies under the appearances of consecrated bread”) with something of his own conjecture (“This bread was consecrated”). So the act of faith itself, strictly speaking, does not assent to anything false.
But there are two things to note about St. Thomas’s response, things he does not draw out. First, the possibility of combining faith with conjecture rests on what he said in Article 2, namely that faith—being an act of the human mind—necessarily involves dividing and combining truths.
Second, once faith begins combining truths in language and argument it is inevitable and even good that it combines truths of faith with human opinions. There is no clean wall between faith and conjecture such that the truths of faith could be combined only with other truths of faith. So even though the act of faith itself, strictly taken, is not about our conjectures, the combining of faith with conjecture is a necessary part of the life of faith taken broadly. When faith concerning the Eucharist gets to follow its impulses, it necessarily gives rise to the conviction that Christ is present here, today, on this altar—it would bizarre if it did not. And in general, the act of faith strictly taken is always bumping into our own conjectures: if we believe that Christ’s body is present under the appearances of bread, what do we suppose the human body is? If we believe that Christ assumed a human nature, what do we suppose human nature to be? And so on and so forth.
The conclusion, put most baldly, is this: faith only consents to what is true, but it necessarily involves a broader life of thought and action which includes error.
This is a very helpful conclusion, for my purposes. I began a while back by supposing that theology is what happens when faith gets to follow its own inclinations. Given that faith cannot err, this claim might seem to suggest that theology and theologians will always say true things. But ST II-II.1.3 shows that, if theology is what happens when faith follows its inclinations, then theology will always include some erroneous conclusions.