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Collect: O God who not only brings about the end of glory, but supplies the means of grace, so work within me that I may be disposed to grace so that I will one day be brought into glory, through Christ our Lord. Amen.
In the Sed Contra, St. Thomas quotes a text from St. Augustine as an authority.
In the Respondeo, St. Thomas briliantly argues for the principle by way of an a fortiori argument. It has already been demonstrated above that merit is in relation to glory. Now, grace is the way to glory, therefore it must be in relation to grace. To illustrate, let's say you took a vacation to Boston. The "last term" of your trip is Boston (obviously). Now, the movement has a formed relation to Boston. Yet, in our trip to Boston, we have many stops along the way. The movement is going to relate to the trip as much as the end. Now, our "trip" to heaven is by way of grace, therefore if merit attains glory, it must attain grace.