Hilary of Poitiers' commentary on Matthew was recently translated into English by D.H. Williams (Commentary On Matthew [Washington, D.C.: The Catholic University of America Press, 2012]). Williams, a patristic scholar who's a Protestant, thinks Paul's concept of justification through faith alone is found in Hilary's commentary. Hilary uses the term "faith alone" in some places when describing his view of justification, and some of his other remarks suggest the same concept without using that terminology. However, elsewhere Hilary makes comments that are suggestive of baptismal regeneration or some other form of justification through works.
Disputes over justification in the church fathers involve several issues. I want to outline some of those before going on to discuss Hilary's significance.
One of the issues involved in disputes over justification in the fathers is scholarly opinion. It's often suggested that no patristic scholar or other scholar with relevant credentials has believed that any father advocated justification through faith alone. Supposedly, there's a scholarly consensus, perhaps even universal agreement among scholars, that sola fide isn't found in the fathers. In that context, the opinion of a scholar like Williams is relevant, regardless of whether his opinion is considered a correct one.
Another issue is terminology. Is the phrase "faith alone" affirmed in any of the fathers? One reason why terminology is important is illustrated by how Roman Catholics and other critics of Protestantism use James 2:24. It's often suggested that the phrase "not by faith alone" in that passage is sufficient to refute a Protestant understanding of justification. But if that phrase, by itself, is enough to disprove Protestant soteriology, then wouldn't an affirmation of the phrase "faith alone" in Hilary or any other father be sufficient to prove that the father in question believed in justification through faith alone? There's also a translational issue involved here. When somebody like Martin Luther uses the phrase "faith alone" in a Bible translation, it's often suggested that the translation is inappropriate. But if the fathers use that terminology, then the argument that it shouldn't be used is weakened, and any claim that nobody used the terminology before Luther is refuted.
Then there's the issue of whether any of the fathers believed in the concept of sola fide, regardless of what terminology was used. And a father could believe in it consistently or inconsistently. I've argued elsewhere that the concept is found in the fathers and in other sources before the Reformation. The fact that the concept can be found in a source inconsistently is important to keep in mind. People are often inconsistent. And patristic and other relevant scholars will often refer to how a father is inconsistent on an issue. Some issues are complicated, controversial, or difficult in some other way that makes inconsistency more likely. The fathers, like people in our own day, were often inconsistent on free will, eschatology, soteriology, and other matters. The fact that a father contradicts justification through faith alone in one passage doesn't necessarily prove that he didn't affirm it elsewhere. If singling out a passage inconsistent with sola fide is sufficient to prove that a father couldn't have believed in justification through faith alone, then wouldn't the reverse be true as well? Couldn't we just cite a passage that seems to affirm sola fide, then conclude that sola fide couldn't be contradicted by that father anywhere else? If that's a simplistic approach to take, and it is, then so is the reverse. We should try to harmonize a writer's thoughts up to a point, but sometimes the best explanation of the evidence is that the source in question was inconsistent.
I don't know enough about Hilary of Poitiers to make much of a judgment about his views as a whole. I've read the entirety of his commentary on Matthew, but not much else from his writings. Judging by his Matthew commentary, he seems inconsistent about justification. At a minimum, Williams can be included in the list of scholars who see sola fide in the fathers, and the "faith alone" term is found in Hilary, multiple times. So, Hilary is at least relevant in those contexts. I'd argue that he also advocates the concept of sola fide, not just the terminology, in some places. For example:
"It disturbed the scribes [in Matthew 9:3] that sin was forgiven by a man (for they considered that Jesus Christ was only a man) and that he forgave sin, for which the Law was not able to grant absolution, since faith alone justifies….In other words, the younger son [in Matthew 21:30] professed obedience, although he did not then bring it to realization because faith alone justifies. For this reason publicans and prostitutes will be first in the Kingdom of heaven….By doubling of the money [in Matthew 25:22] we find that works were added to faith. What one believes in his mind, he accomplishes through deeds and actions." (Commentary On Matthew, 8:6, 21:15, 27:8)
Not only does the phrase "faith alone" suggest that only faith is involved, but so does the context of Hilary's use of the phrase. The Matthew 9 and 21 passages don't involve works on the part of the person in question. That's one of the reasons why Protestants so often cite the Matthew 9 passage in discussions about justification. Furthermore, Hilary tells us that the son in the Matthew 21 passage hadn't yet practiced obedience. So, it doesn't seem that Hilary is including works resulting from faith in his definition of faith at that point. In the third passage I've quoted above, Hilary refers to works being added to faith and how faith is something that occurs "in his mind". Again, it doesn't seem that works are being included in Hilary's definition of faith. Rather, he's referring to something that occurs inwardly ("in his mind") that later has works added to it. In another passage (20:7), not quoted above, Hilary refers to how the workers in Matthew 20:7 were justified as a "gift", "freely", etc. apart from "work rendered". Again, the best explanation seems to be that Hilary is thinking of faith by itself, not faith accompanied by works of some type.
Elsewhere, however, Hilary seems to make comments that are inconsistent with the ones quoted above. He refers, for example, to how "When we are renewed in the laver of baptism through the power of the Word, we are separated from the sins that come from our origin, and are separated from its authors….Through the gift of baptism, he [Jesus] endows with the grace of salvation those who were his detractors and persecutors." (10:24, 18:10)
Perhaps Hilary held something like a Lutheran view of justification or some variation of some other Protestant view. I suspect he was inconsistent to some extent.
Here's some of what Williams writes about Hilary and justification:
"Throughout the Commentary there is an unambiguous emphasis on the theology that stems from Paul's epistles, especially with regard to the concept of being justified through faith. Here, some sixty years before Augustine and lacking the Origenist contributions, Hilary follows little or no precedent. About twenty occurrences of the phrase fides iustificat (or fidei iustificatio) are to be found expressing an interpretation germane to an understanding of the Gospel. It would seem that Hilary's interest involves much more than mere restatements of Pauline passages. In fact, Hilary is the first Christian theologian explicitly to have formulated what Paul left implicit by referring to God's work of grace in the phrase fides sola iustificat: 'Because faith alone justifies…publicans and prostitutes will be first in the Kingdom of heaven' (Mt 21.15)." (30-1)