NOTES for Act 2:1
As we continue reading the book of Acts, we notice that being of one accord is one of the fruits of the Holy Spirit, who descended on the apostles at Pentecost and then worked powerfully in the Church. But here, in this verse, it becomes clear that this one accord among the apostles arose even before the descent of the Holy Spirit, and there is no contradiction here, but a living spiritual dynamic. If there had been no unity of heart among the disciples, Pentecost would perhaps not have taken place; yet they receive their true, genuine, and unbreakable unity of heart at the moment when they see the "tongues of fire" descending on each of them.
Where did this "preliminary" unity of heart come from? Did Pentecost take place only because several people happened to have the same mood and thoughts? Of course not. These are not random people, but Christ's disciples, whom He Himself had formed for three years; these are the very disciples to whom He appeared after His Resurrection and "opened their minds to understand the Scriptures"; and, finally, these are the very apostles to whom He said, breathing on them, "Receive the Holy Spirit." So even this "small" unity of heart is the result of God's work.