Blessed are the merciful, for they will be shown mercy. Very much like the golden rule. Do unto others as you would have them do unto you. Except in this case, the one doing unto us is God. So if we show mercy to other people, then God will show mercy to us.
Like faith and believing is Jesus, action is an integral part of mercy. We’ll see that just saying, “God have mercy on you”, isn’t what Jesus is talking about. Nor is praying for someone and walking away.
It’s about doing something. Helping. kindness or good will towards the miserable and the afflicted, joined with a desire to help them.