"Blessed are the merciful, for they shall obtain mercy."
The beatitudes are not a checklist designed for earning or achieving, as if those who tend toward withholding the punishment deserved by others, will as a result, be rewarded with the blessing of receiving mercy themselves.
The beatitudes identify blessings that have already been received. First, God's Grace in Jesus Christ causes believers to recognize that they have already received mercy and they progressively become merciful themselves. Second, when standing in the final judgement, they will obtain mercy - because the judgement they earned, has fallen on Christ. Blessed indeed!
Matthew 5:3-10
Blessed are the poor in spirit, for theirs is the kingdom of heaven.
Blessed are those who mourn, for they will be comforted.
Blessed are the meek, for they will inherit the earth.
Blessed are those who hunger and thirst for righteousness, for they will be filled.
Blessed are the merciful, for they will be shown mercy.
Blessed are the pure in heart, for they will see God.
Blessed are the peacemakers, for they will be called children of God.
Blessed are those who are persecuted because of righteousness, for theirs is the kingdom of heaven.