Acts 12:24-25 meaning

This chapter began with persecution against believers in Jesus, and it ends with a hopeful turn, showing that God's will cannot be stopped by men. Luke just described the ultimate fate of Herod Agrippa I, who was struck dead by God, and goes on to demonstrate that God's plans continue to prosper:

But the word of the Lord continued to grow and to be multiplied (v. 24).

The word of the Lord means God's message, what God wants to tell humans. In Acts, the word of the Lord largely refers to the Gospel, the good news about how we can be born anew and be saved from sin and death if we have enough faith to look upon Jesus, to believe His death and resurrection will save us from the deadly venom of sin (John 3:14-15). It also refers to God's message that we can be restored to God's original design to work together in harmony with God and one another when we walk by faith, in the power of the Spirit (Galatians 5:16-22). 

So, the good news about Jesus continued to grow and to be multiplied (v 24): people continued to believe in this good word from the Lord! The church grew, the number of believers grew. For many years now, beginning with Jesus, the world had been trying to destroy this message by killing those who spoke it (Matthew 27:20, John 19:15, Acts 5:33, 7:59, 8:3, 12:2), but nothing can prevent God's plans.

Luke catches us up with Barnabas and Saul (later known as Paul, the Greek version of his name). The last instance they were mentioned, they were journeying down from Antioch to Jerusalem to deliver "a contribution for the relief of the brethren living in Judea" (Acts 11:29). 

Prophets had come from Judea to Antioch (in modern-day Turkey), warning that a famine would happen in the near future. So the Antiochian believers, who were better off materially than the Jewish believers, raised money from their own pockets to send to Judea, to help prepare their poorer brethren for the coming hard times. Barnabas and Saul (Paul) were entrusted with this charitable donation, and carried it all the way back south to Jerusalem. 

Luke tells us nothing about their time in Jerusalem. It is unknown whether they were there while Herod beheaded James the Apostle and arrested Peter (Acts 12:1-3). But based on the order in which Luke has arranged the events of this chapter, it comes across that they arrived in Jerusalem after the persecution of the church. And it does not sound as though they stayed very long:

And Barnabas and Saul returned from Jerusalem when they had fulfilled their mission, taking along with them John, who was also called Mark (v. 25).

They seem to have been focused only on the delivery of the money for the Jewish believers, because they depart Jerusalem as soon as they had fulfilled their mission. Their mission was to hand the Antiochian monetary gift to the elders in Jerusalem for distribution. Barnabas and Saul (Paul) leave Jerusalem, taking along another believer to help them with their gospel-preaching. They took John, who was also called Mark. Luke briefly introduced us to John Mark earlier. It was in his mother Mary's house that the group of believers had gathered to pray for Peter's life while he was in prison:

"the house of Mary, the mother of John who was also called Mark, where many were gathered together and were praying."
(Acts 12:12)

John Mark was a cousin of Barnabas's (Colossians 4:10). According to early church tradition, he is the author of the Gospel of Mark, which was based on the testimony of Peter.

The 4th-century church historian, Eusebius, quoted a 2nd-century Christian writer, Papias, who said, "Mark, being the interpreter of St. Peter, wrote down exactly whatever things he [Peter] remembered, yet not in the order in which Christ either spoke or did them; for he [Mark] was neither a hearer nor a follower of our Lord, but he was afterwards a follower of St. Peter."

Here in Acts 12 there is evidence that Peter knew John Mark. Given that his mother Mary hosted the prayer gathering at her house, it is possible that her house was a common location for believers to gather together. In the early days of the church, believers met in people's homes, rather than having a building designated for assembly. The Greek word translated "church" means "assembly" and refers to the gathering of the people rather than the physical space in which they gather (as is a common use in modern times). 

Peter appears to have known Mary and John Mark. Years later, toward the end of his life, Peter wrote his epistles, or letters, and references Mark, who is with him in Rome, assigning to him a title which indicates a close friendship, "my son,"

"She who is in Babylon, chosen together with you, sends you greetings, and so does my son, Mark."
(1 Peter 5:13)

Acts 12 provides the first mention of John Mark in scripture. He will accompany Barnabas and Saul (Paul) back to Antioch, and from there the trio will embark on a missionary journey into the western world, the first of its kind, to spread the good news of Jesus Christ. John Mark will desert the mission and become a source of dispute that divides Barnabas from Paul. Accordingly John Mark will become yet another example of a follower of Jesus who overcame failure to do great things for the gospel of Christ.