Aquinas wrote a tremendous amount of material on the subject of the Eucharist. This particular snippet, in isolation, isn't truly indicative of whether he leaned more toward an Anglican view or more toward a Roman (a la Trent) view. I think Ambrose's writing was misconstrued to some degree by Aquinas and the RCs. Let's break this statement apart and look at it. Ambrose says there is only one victim because there was a one-time-only sacrifice of Jesus Christ. He then says, "this latter sacrifice is a pattern of the former." Ambrose wants to call the Eucharist a 'sacrifice', but states that it is (only) a 'pattern' of Christ's sacrifice. 'Pattern' might be likened in our times to a photo copy, but in his day it would be a tracing or some other duplicate likeness of the original. Note that if it's a pattern, it is not the one original, correct? And then, what does Ambrose state in his next sentence? He says that all of the Eucharists offered everywhere are "but one sacrifice." This statement mixes up the Romans. The problem is, RCs interpret this to mean not merely that all Eucharists are "one sacrifice" (that sacrifice being a pattern or copy of Christ's one sacrifice), but instead they conclude that all Eucharists are a continuation of Christ's one sacrifice; i.e., more than a pattern. Instead of being a picture of the one-time sacrifice, they hold Eucharist to be the sacrifice itself. This contradicts Ambrose's statement that "Christ was offered but once," for they continue to offer up Christ daily in a perpetual prolongation of His sacrifice as if it were not finished. (Christ said, "It is finished," and He now sits at the Father's right hand interceding for us but no longer sacrificing Himself for us.)
I was referring specifically to Aquinas' teaching on the Eucharistic sacrifice. No one would suggest that we ignore what Aquinas says in the Pars Prima on the nature of God just because he wrote voluminously on the subject elsewhere, nor should we do so here. The Summa Theologiae was the definitive statement of his theology and the (unfinished) apex of his life's work, after all. The bulk of Aquinas' writings on the Eucharist, both in the ST and in the Contra Gentiles (the latter of which doesn't mention the sacrificial aspect of the Eucharist at all), revolves around (1) the reality of Christ's presence in the sacrament, and (2) the nature of the action that effects that reality. Aquinas is there clearly at variance with historic Anglican doctrine, relying as he does on outdated Aristotelian metaphysics. But his teaching on the Eucharistic sacrifice seems much closer to Anglicanism than it does to post-Tridentine Roman Catholicism. The apparent incoherence in St. Ambrose disappears, as C.B. Moss pointed out in The Christian Faith, once one recalls that sacrifices as such have three stages: (1) immolation, (2) oblation, and (3) consumption. In the case of Christ, the immolation happened at Calvary, the oblation is happening perpetually in heaven, and the consumption happens every time the Eucharist is celebrated. The many sacrifices are one because of the identity of the Victim.
What's fascinating in these snippets is that Aquinas does not teach of the eucharistic sacrifice as propitiatory. In this he accords with our doctrine of the eucharistic sacrifice. And he also is one of the last adherents of the Patristic doctrine of eucharistic sacrifice (as non propitiatory). Soon in the middle ages, you will begin to see a revision of theology, and the alteration of the Sacrifice doctrine to indicate the notion of propitiation. Thus by the time of Trent, the Roman church will insist (on pain of death) that the Mass must be done for the propitiation of the quick and the dead. But Aquinas is one of the last (before the Reformation) who still represented the patristic doctrine.
@Stalwart you may find this interesting. The whole theory of mystical immolation actually postdates even Trent itself. http://cdn.theologicalstudies.net/61/61.2/61.2.2.pdf
Yes, and it's easy to see how the Romans arrive at that erroneous concept. Once they started thinking that the Eucharist is a continuation of the one sacrifice (making it an ongoing thing), the propitiatory aspect of Jesus' actual suffering and death is extended in their minds, and thus the sacrifices of their Masses become propitiatory in nature (in their theology, not in actuality).
Another way they made the mistake, was they assumed that "sacrifice", the word itself, is another word for "propitiation" or "atonement". I've seen many of them argue, that when one sees the word "sacrifice" in the church fathers, that that is patristic proof of propitiatory mass. Why else would one sacrifice, if not for propitiation? Isn't the notion of priest and sacrifice basically analogous to an aztec or greco-roman priest sacrificing a victim, for heavenly propitiation? That's literally how they understand the word sacrifice.