Mounce is not correct at this point. In Greek, the neuter demonstrative (
touto) by no means always corresponds in gender with the antecedent. Both
charis, 'grace' and
pistis, 'faith' are feminine, so
touto does not agree in gender with either of them.
Touto does not refer to faith precisely, but to the fact of our exercising faith. If Paul was saying, 'by grace you have been saved, and that being saved is not of yourselves,' it would have been a tautology: Grace is never of ourselves; it is always of God. He might possibly have meant that the whole thing, grace and faith, are the gift of God, but that will not help
@Silverhair.
Pistis is the nearest antecedent and therefore is the most likely candidate.
There is a massive amount of literature on this subject. If anyone has time to waste, grab two or three commentaries and away you go.