Faith is a gift from God.
Eph 2:8 "For by grace you have been saved through faith; and that not of yourselves, it is the gift of God;"
If God looked down the "time tunnel" to see their faith, the only thing He would see is the gift He had given them.
The ít'is not referring to the faith, but to the grace.
It is not faith, but the grace plan of salvation that is the gift.
Below is from Calvin's Commentaries,
. For by grace are ye saved. This is an inference from the former statements. Having treated of election and of effectual calling, he arrives at this general conclusion, that they had obtained salvation by faith alone. First, he asserts, that the salvation of the Ephesians was entirely the work, the gracious work of God. But then they had obtained this grace by faith. On one side, we must look at God; and, on the other, at man. God declares, that he owes us nothing; so that salvation is not a reward or recompense, but unmixed grace. The next question is, in what way do men receive that salvation which is offered to them by the hand of God? The answer is, by faith; and hence he concludes that nothing connected with it is our own. If, on the part of God, it is grace alone, and if we bring nothing but faith, which strips us of all commendation, it follows that salvation does not come from us.
As Calvin notes, we bring the faith, which adds nothing to the grace, but only accepts it.