Faith is not merely a generic "trusting in Jesus". First, faith includes believing a set of propositions, i.e. the whole truth God has revealed to the Church, the "one faith" which we profess in the Creed, and which the catechumen confesses before the Church just prior to being baptized (CCC 1237). Second, faith also includes the act of receiving the sacraments, and trusting what the Church says about them. The person who claims to have faith but refuses the sacraments does not have faith. As the Catechism says, the "response of faith" is "inseparable from Baptism". (CCC 1236) Third, faith also includes trusting those through whom the propositions and sacraments have come to us, that is, the magisterium of the Church. This is what St. Augustine means when he says, "For my part, I should not believe the Gospel except as moved by the authority of the Catholic Church." And this is what St. Cyprian means when he says, "He can no longer have God for his Father, who has not the Church as a mother." In this way, true faith includes all three bonds of unity. We often seem to interpret St. Paul's statements about "justification by faith" as meaning that we are justified by a generic "trusting in Jesus". But the faith of which St. Paul is speaking is not a mere "trusting in Jesus", but a rich full faith that includes all three aspects mentioned above. That is, in fact, the fullness of what it means to "trust in Jesus".
Since faith is something thicker and fuller than a merely subjective and inward "trusting in Jesus", we do not need to treat "justification by faith" and "justification by baptism" as mutually exclusive. Peter Leithart writes:
This is, I think, what Paul means in Romans 6:7 when He says - in a context having to do with deliverance from the power of sin, and, not incidentally, with baptism - that we are “justified from sin.”
The fuller conception of faith that St. Paul and the Church hold allows us to read the epistle to the Romans without worrying that "justification by baptism" intrudes upon or detracts from "justification by faith". Being baptized is part of what it means to have faith, and so being justified by baptism is part of what it means to be justified by faith. The entirely modern, entirely individualistic, entirely subjective, entirely internal and entirely phenomenological conception of faith that is common in American evangelicalism is not the faith of St. Paul or the Church. Recovering the faith that was handed down to the saints will help us overcome the individualism that presently hinders the reunion of all Christians.
"I felt the necessity to write to you appealing that you contend earnestly for the faith which was once for all handed down to the saints." (Jude 1:3)
UPDATE (09/23/07): I have modified this post. My original comments here were critical of Leithart's comment, but a gracious reader pointed out that I had misread him, and that he was actually saying just the opposite of what I had thought. So I am glad now to be able to retain the quotation as a positive example of this fuller conception of justification.
"Let unity, the greatest good of all goods, be your preoccupation." - St. Ignatius of Antioch (Letter to St. Polycarp)