Albert Barnes Commentary Matthew 5:10

"Blessed are they that have been persecuted for righteousness` sake: for theirs is the kingdom of heaven." — Matthew 5:10 (ASV)

Persecuted. To persecute means literally to pursue or follow after, as one does a fleeing enemy. Here it means to afflict or oppress someone on account of their religion. Those who persecute others injure their names, reputation, or property, or endanger or take their lives, on account of their religious opinions.

For righteousness' sake. Because they are righteous, or are the friends of God. We are not to seek persecution. We are not to provoke it by unusual beliefs or conduct, by violating the laws of civil society, or by ways of speaking that are unnecessarily offensive to others. But if, in the honest effort to be Christians and to live the life of Christians, others persecute and revile us, we are to consider this a blessing. This is in itself evidence that we are the children of God and that He will defend us. All that live godly in Christ Jesus shall suffer persecution (2 Timothy 3:12).

Theirs is the kingdom of heaven. They have evidence that they are Christians and will be brought to heaven.