This is what I think.
In the world of the Dresdenverse and limiting the definition to the arcane aspect only, a saint is probably someone who regularly, consciously and directly channel the power of higher being without actually being invested with a mantle. Which is entirely different from faith magic.
Faith magic, as I understand it, is the result of humanity's willingness to have faith. It is the power that empowers thresholds and prevents ghost from crossing over the cemetary wall. It is actually just another form of magic aside from wizardry. The difference from channeling a higher power is this. People believes that being inside one's home is safe, and therefore the house has a threshold. By believing that the cemetary wall separates the living from the dead, the wall gains the power to resist ghost. There is no higher power involve. The belief of that many humans simply translate into power.
Harry has a mantle and soulfire is invested directly into him similar to a mantle, so he is not a saint but a knight. Michael and the other KoTC channel power via an artifact, so they are not saints either. Murphy channel an angel in book 12, but it is not voluntary, so instead of a saint it is more like she is being possessed. LTW's shapeshifting probably falls into the same line as Harry's soulfire investiture, so using my definition LTW is not a saint either.
Ft. forthill though is probably the closest to a saint we see on screen. In his blessings, Ft. Forthill can invoke power. He do it regularly and conciously. Actually, in book 3, when Michael face the rampires without ammoracchius, Michael invoke the same power, but since it is a one time deal and not regular he may or may not qualify. Oh, scratch it, Michael is a saint. He invoke that power in book 3, book 10 and book 15, especially in book 15 when he chase away Tessa. 3 times is regular enough for me.
In short, saint are a human / mortal who channels the power of higher beings. They channels power, but not invested with power.