Steven Paulson recently gave a talk where he discussed how Augustine characterized prayer in a similar manner. Faith prays to God for sustaining grace in order to make the distance. I must be humble and I cannot be certain that I will endure to the end. Therefore I must constantly pray that God give me sustaining grace. Notice that the Reformed also think of the life of faith as something that needs to be "sustained" through the "perseverance of the saints"(note the P in TULIP). This is true for the Reformed, even though they agree with Luther against Augustine on the total assurance of salvation through Christ.
Nevertheless, Paulson states, when faith primarily is expressed through prayer, then it always rest on uncertainty. Why? Because prayer is always prayer for something. Praise does of course occur in prayer, but usually in the context of asking for something. Praise of God when alone is generally characterized as being just that, praise and not prayer. If prayer is always asking for something, then it means that it rests on uncertainty. It humbly asks "if" God will do such and such.
For this reason, Faith does not rest on prayer and prayer is not a means of grace. Faith looks to Word and sacrament wherein the promise made by Jesus on the cross is already "yes" and "Amen." Faith is based on certainty. It does not have to look ahead for preserving grace (as both Augustine and the Reformed hold), but it looks back to baptism. Baptism means that my new being of faith coram Deo has already been actualized through dying and rising with Christ. Everything is done and over. There is not uncertain, because the deed is done.