What does Matthew 5:12 mean?

Jesus has declared that those treated badly on His account are "blessed" (Matthew 5:11). Abuses they could expect were insults, persecution, and false accusations about doing terrible things. The context of these attacks is when people are truly following the teachings of God—the world hates them specifically because they emulate Jesus (John 15:21).

Now Jesus explains why willingness to be connected to Him despite harsh treatment is a blessing. Those who do so will receive great rewards in heaven. Jesus does not hold this out as a consolation prize for a difficult life. He tells His disciples to rejoice and be glad about the opportunity to be treated awfully for His sake. The rewards waiting in heaven because of this honor (Acts 5:41; 1 Peter 4:12–13) makes the experience itself a victory worth celebrating.

While Jesus will later predict intense hatred levied against Christians (John 16:1–4), attacks on God's people are not new. God sent many prophets to Israel to deliver messages to the people. Many of those prophets were persecuted for representing God. Now many of Jesus' disciples will be persecuted for representing Him to the world, which will earn them great rewards in heaven, as well.

For the first time, Jesus mentions heaven as the destination of His followers instead of the kingdom of heaven (or kingdom of God). He also describes the blessing of those who are His as including more than just entry into the kingdom, but rewards in heaven.