The fact that Christ speaks regularly and with vivid imagery of faith as having dimension (it can be little or great and can grow) and even as having material properties (it can be the size of a grain of mustard) leaves some imagining that faith by its nature is an objective substance that must be routinely secured extra nos (i.e., from without us). This is not to say that they view faith as a physical substance; still the imagery is strong and the potential for confusion is great. Many, in my experience, pray for faith with the expectation that God will miraculously and externally insert some faith “substance” into the mind.
This is a fundamentally incorrect understanding of the sanctification process. While it is true that saving faith comes to us suddenly as part of the regenerate self, the faith we receive in the new birth is dispositional—it is a part of the new man and thus intra nos. In regeneration God does not create a tabula rasa, iteratively add faith like so much water, and then wait for responses of faith to sprout; rather he creates a believer equipped with everything necessary for life and godliness. And from the point of new birth forward, faith grows within the believer as he cultivates and nurtures his own faith subjectively through means.
Comparison might be made in the physical realm to the idea of strength or health. Every person is endowed with a measure of physical strength and health and the capacity for greater strength and health. If I want to become a stronger or healthier person, I might legitimately pray for these things, but in praying I would not be asking God to miraculously “add” strength and health to me, but rather that he would help me to make the proper and disciplined use of means to become stronger and healthier. This is how providence manifests in the material realm.
In the same way, we are most certainly to pray for increased faith (Luke 17:5), just as we pray for increased wisdom (Jas 1:5), peace (2 Thess 3:16), love (Phil 1:9), knowledge (Col 1:9–10), and a host of other virtues. We should not pray for these things, however, anticipating fresh and miraculous infusions of these virtues extra nos; instead, we should pray for the providential cultivation of these virtues intra nos through the use of the sundry means necessary to their increase. For instance, faith grows as we cultivate proper notitia, assensus, and fiducia; wisdom as we consult the Word and godly counsel; and peace as we practice obedience. And so forth. We pray for them, most assuredly, but never in isolation from the means necessary to their achievement.