St. Thomas on Justification - Baptism confers justification ex opere operato

Nothing terribly surprising in this post, which I include more in the way of completeness than anything.

Writing about satisfaction, St. Thomas says that Baptism confers justification ex opere operato – that is, "in virtue of the deed." But it is not man's deed; rather, it is God's.

Baptism confers justification in virtue of the deed [ex opere operato] which is not man's deed but God's... [ST Supp., Q14, A3, ad 2]

In what way, then, does this order of things suggest that we believe that we save ourselves by our works? It doesn't. Hence those who say otherwise would seem to be misinformed.