Quotes all from his Galatians Commentary:

"For faith liberates, and anyone, as we have said, who hopes for help in any way besides Christ, even if it be along with Christ, does not have faith." (Gal 5:9)

"For faith itself alone grants justification and sanctification." (Gal 2:16)

"All of those who stand on the basis of faith, then, are among the children of Abraham, as I have often pointed out. The entire Mystery, which was enacted by our Lord Jesus Christ, requires faith alone....This means that our whole life will be accounted to us as justice." (Gal 3:7).

"[The Law] was not given for life to be sought from it, but so that it might in its scriptural form teach about all the sins and show how they are to be avoided. So righteousness is not based on the Law—that is, neither justification nor salvation come from there but are based on faith, as was promised." (Gal 3:21)

"For the whole power of the Mystery has worked to this effect: that an indulgence of sins would come about for us through the grace and mercy of God, and that eternal life would be supplied, as we have often taught, on the basis of God’s grace, not works or merits. But this happens through the Spirit. On the other hand, when one hopes for justification on the basis of one’s works, the hope is not based on the Spirit. Hope based on the Spirit is what we await, and this is what it means to follow the gospel of Christ" (Gal 5:5).

"He [Paul] aims to prevent the Galatians from…believing that as long as they retained faith in Christ, something further could still be advantageous for them, if they would perform something based on works as well. To the contrary, the apostle denies that any blessing comes about on the basis of works; he states rather—and this is even more serious, and opposed to a blessing— that those who carry on their lives based on works are under a curse ."(Gal 3:9)

"For all things come about on the basis of faith: the promise was given to Abraham based on faith, and thus to his seed as well." (Gal 3:20)

"If one is given the name son, according to the previous discussion, one is also an heir, not, though, by things done or by one’s works, rather by the mercy and grace of God. This is Paul’s implication, that it is rather through God, just as has been demonstrated in many passages: that it is not of the one who runs but of the one who shows mercy, and that all things are through the grace of God." (Gal 4:7)

"Thus they came to agree even on this matter: that the hope of salvation is not to be found in our performing works for the poor. But just what are they admonishing Paul and Barnabas to do? That we would be mindful of the poor. Not that we should expend all our efforts on this, but that we would provide for those who do not have what is within our means. They admonished only that we should be mindful of the poor, not that we should put our concern, our worries, and all our strength into this matter, as if holding on to our salvation depended upon it." (Gal 2:10)

"[If I] would believe myself to be justified based on the works of the Law, that would be ungrateful to the one who did so much for me, who for my sake would put himself in the line of fire, in order to liberate me from my sins by taking their penalties upon himself." (Gal 2:21)

"Therefore those who stand on the basis of faith will be blessed with faithful Abraham (3: 9). This is what they will be blessed means: it will be accounted to them as justice, and they will be blessed by God. Now, they will be blessed by God means to have been justified. And to be justified means to be freed from the law of servitude. Therefore, those who will be blessed on the basis of faith will be blessed because Abraham was. So faith is everything." (Gal 3:9)

"Therefore those who stand on the basis of faith will be blessed with faithful Abraham (3: 9). This is what they will be blessed means: it will be accounted to them as justice, and they will be blessed by God. Now, they will be blessed by God means to have been justified. And to be justified means to be freed from the law of servitude. Therefore, those who will be blessed on the basis of faith will be blessed because Abraham was. So faith is everything. Because the promises guarantee inheritance and justification, as long as this Law which is based on works calls souls away from faith, as long as it keeps souls occupied in some other thing (resulting in their looking for the promises not based on faith but on something else, that is, on works), the Law is opposed to the promises." (Gal 3:21)

The Bible is the sole authority so this doesn't really prove anything, but many people will try and lie and say "faith alone was invented in the 1500s by Martin Luther", so this is just the easiest way to 100% easily prove that they are lying (besides just showing them the Bible). Marius Victorinus was extremely popular, he was mentioned by Jerome, Augustine and many others. He taught that faith alone saves, not works, just like the Bible does.