What does Mark 4:32 mean?
The mustard seed is not the smallest seed in all of existence, but at one to two millimeters, it was the smallest seed the Jews planted in their gardens at the time of Christ. It grows quickly into bushes nine feet tall and, if left to its own devices, becomes an impenetrable thicket.The reference to the birds that shelter in the mustard sounds random, although poetic. But Jesus is referring back to Old Testament prophecy in which birds that nest in trees represent representatives of different nations who find protection in powerful leaders. In Ezekiel 31:2–6, that leader is Pharaoh; in Daniel 4:20–22, it's Nebuchadnezzar; while in Ezekiel 17:23, it is God. The first few chapters of Acts bear witness to what is essentially Jesus' prophecy. In Acts 1:15, right after Jesus ascension, about 120 people in Jerusalem claim to be Christ-followers. The Holy Spirit comes, and Peter preaches to a crowd of Jews from all over the Roman Empire (Acts 2). Three thousand from among the crowd ask to be baptized (Acts 2:41). Before long, five thousand men and an unnamed number of women are saved (Acts 4:4).
This massive growth isn't the result of Peter and John's great teaching, but of the Holy Spirit working in hearts (Mark 4:26–29). Peter and John are merely faithful to Jesus' command to teach what they had been taught.