Whether the Lord visited Nephi in person or touched his heart by the power of his Spirit (see Alma 17:10) is not known. The result, however, was the same—Nephi’s heart was softened, and he gained a personal witness, born of the Holy Ghost, that the testimony of his father was genuine and true.
With this anchor to his soul, Nephi was able to serve as a knowledgeable and competent co-witness and testator with Lehi and was also able to bear up under the trying and demanding rigors of the journey ahead. (It is infinitely easier to handle the “how” when we know the “why.”)
Nephi and those like him should never be labeled as blindly obedient; Nephi was obedient with his eyes—temporal and spiritual—wide open. On this occasion Nephi came to know by revelation that what his father had been teaching was true; shortly, after making a humble petition and request, he received a similar revelation himself (see 1 Nephi 10:17-14:30).
How could a believing heart become more believing? The soil of faith becomes richer and more fertile with each spiritual experience, the harvest of its fruits more plentiful.