So in other words, you do the work of believing because God gave you the necessary grace to do so, and while God gives you eternal life because you do the work of believing, God still gets the glory because it is he who gave you the grace to do that work?
This is a really good point when making the case against certain sorts of Protestants for whom a believing is the criteria or condition upon which the gift of eternal life is given.
Fortunately, Lutheranism recognized this error right up front, and both the Apology of the Augsburg Confession and the Formula of Concord condemn this understanding of the role of faith in justification, and that for two reasons.
First, because faith as something
intra nos (internal to us), that is, as belief, is the Catholic definition of faith (faith as intellectual assent to the truth of Christianity), and Lutheranism has always denied that this is a comprehensive definition of faith. Justification and salvation are
always, for Lutherans at least,
extra nos (eternal to us).
By way of explanation to this first point, allow me to point out a subtle but important distinction between the definition of faith in Lutheranism and Calvinism. In both, saving faith consists of
notitia (knowledge of the truths that Christianity teaches) and
assensus (assent to those truths of the faith), just as in Catholicism, but it also consists in
fiducia (fidelity to the faith, which is the reason faith produces good works). Lutherans and Calvinists hold to this threefold definition of faith over-against the twofold definition of Catholicism.
However, in Calvinism, the subtle priority is placed on the assent of faith (
assensus), and thus Calvinists who baptize babies have long struggled to argue that babies can possess intellectual truth-content (and their inability to sufficiently prove this point is basically what led to the English Baptists).
Fiducia grows out of
notitia and
assensus. Yet in Lutheranism, the priority has been placed on
fiducia, which is a sense of existential trust which we know babies possess even in the womb.
Fiducia is not complete without
assensus, of course, but it is
fiducia that allows
assensus to happen in the first place.
This leads to the second point: in Lutheranism, saving faith is primarily
not belief, but
the situation into which we are placed by grace. Think of a baby being held by its mother; that baby is
not "doing" trust; it is in a position of trust that is solely dependent on the work of the mother in maintaining the necessary arm strength to hold the baby aloft. That infant my squirm out of her arms (Lutherans have never believed in eternal security of either the eternal perseverance or once-saved-always-saved varieties) and so fall, but the mother will always strive to pick the infant up again, and that picking up can never be a credit to the infant.
We trust in God because we are in his arms, and we are in his arms because he has picked us up and carries us. And it is that
situation into which grace places us that produces
both knowledge (
assensus) of the one who is carrying us (what Catholics think of in terms of "faith" or "mere belief")
and which generates in us a love for that one who is carrying us and leads us to love others as well (
fiducia).
