In his new encyclical Spe Salvi, Pope Benedict XVI discusses the verse in the Epistle to the Hebrews: “Faith is the substance of things hoped for; the proof of things not seen”. The Holy Father comments:

To Luther, who was not particularly fond of the Letter to the Hebrews, the concept of “substance”, in the context of his view of faith, meant nothing. For this reason he understood the term hypostasis/substance not in the objective sense (of a reality present within us), but in the subjective sense, as an expression of an interior attitude, and so, naturally, he also had to understand the term argumentum as a disposition of the subject…Faith is not merely a personal reaching out towards things to come that are still totally absent: it gives us something.

H.H. Benedict XVI Spe Salvi, 7