Rom. 10:17--"So then faith cometh by hearing, and hearing by
the word of God." (cf. the context, vv. 9-21.) Acts 4:4--"Howbeit
many of them which heard the word believed." In this instance the
_spoken_ word, the Gospel, is referred to; in other cases the
written Word, the Scriptures, are referred to as being instrumental
in producing faith. See also Gal. 3:2-5. It was a looking unto the
promises of God that brought such faith into the heart of Abraham
(Rom. 4:19).
Prayer also is an instrument in the development of faith. Luke is
called the _human_ Gospel because it makes so much of prayer,
especially in connection with faith: 22:32--"But I have prayed for
thee that thy faith fail not." 17:5--"And the apostles said unto
the Lord, Increase our faith." See also Mark 9:24; Matt. 17:19-21.
Our faith grows by the use of the faith we already have. Luke 17:5,
6; Matt. 25:39.