Jesus was coming to the end of His pre-mission briefing to the disciples. It would be a dangerous mission because many, even in their own families, would not understand and could turn against them. However, some people would welcome them (Matthew 10:11-13). But would they also not be put in danger? Maybe, but they needed to be reassured that whoever might be against them, the Lord was for them (Romans 8:31).
 
Disciples, apostles, evangelists (and indeed, all believers) are ambassadors of the King of heaven (Matthew 28:18-20). Those who welcome them because of who they represent, welcome their King. Being willing to engage with believers, to learn from them about Christ, is the first step in becoming another of His disciples.
 
King Jesus is not mean: He is very generous. He will reward everybody who knowingly welcomes His messengers. This is not just the common Middle Eastern courtesy of giving hospitality to travellers (Hebrews 13:2); but is intentionally accepting the people the Lord sends, because they come from Him and speak for Him. Those who welcome God's servants will be rewarded as though they are God's servants themselves ... because they are!
 
It is important to do the right thing, because it is the right thing to do. But, under pressure, doing the right thing is more than a duty; it also shows our heart's desire for the God of righteousness. We do not do what is right because of the reward, however, God promises to reward all those who honour Him, both in this life and in the life to come (1 Samuel 2:30; 2 Timothy 4:8; Hebrews 6:10). In times of difficulty, danger, or uncertainty, when there is nothing positive to see; believers have every right to be confident that their faithfulness will be rewarded ... because Jesus has promised.