There was a time, signified by this day of Pentecost, when the church was unified—perhaps as unified as it ever was in its entire history. These verses reveal two elements of the time when the unity of the church was at its very peak.
This is the epitome of "feeding the flock"—and ALL of the body is participating, not just the ministry. Everybody is nurturing everybody else. The whole body participating in two major things, pursuing the faith once delivered and taking care of each other.
The New Testament epistles make it very clear that later, when the first century church was splitting, the people were counseled to get back to the faith once delivered—which means that they had drifted from it. They were no longer doing the things they were doing in Acts 2. Again, why? Why counsel them to get back to the apostles' doctrines?
Putting this together, asking where faith arises from, there are two major components. The first is God and what He does (I Corinthians 2). He opens up our mind. He predisposes it for us to receive something. The second is expounded upon by Paul in Romans 10. "Faith comes by hearing, and hearing comes by the word of God." Those two work together. What God does, by a miraculous act of His mind, of His will, of His Spirit working in our minds, is combined with the message He gives to the person He sends. It is to be the basis and foundation of our conversion and our faith. From that point on, it becomes a matter of learning more specifically the things that are contained within the message that was delivered to us.
— John W. Ritenbaugh