substitution

from comment 437 here http://www.calledtocommunion.com/2010/04/catholic-and-reformed-conceptions-of-the-atonement/#comment-86421

How then can the work of Christ be truly said to have been performed in our “name” or “stead”?

The answer is in that same paragraph (Trent VI.7):

the final cause is the glory of God and of Christ and life everlasting; the efficient cause is the merciful God who washes and sanctifies gratuitously, signing and anointing with the holy Spirit of promise, who is the pledge of our inheritance, the meritorious cause is His most beloved only begotten, our Lord Jesus Christ, who, when we were enemies, for the exceeding charity wherewith he loved us, merited for us justification by His most holy passion on the wood of the cross and made satisfaction for us to God the Father,

He did it for us, meriting justification for us, making satisfaction for us, so that we might have everlasting life.