"Blessed are the merciful, For they shall obtain mercy." (Matthew 5:7)
Mercy that is achievable by the merciful is not necessarily the same mercy that others show to us when trying to lend a helping hand. Quite often the "tender mercies" of others are in fact "cruel".
God has mercy on us in as much as we have mercy towards our neighbor. This is the same as the principle of of forgiveness, God forgives our mistakes in as much as we forgive the mistakes of our neighbors. (Matthew 6:12)
Being merciful is a new state that arises with the new birth, where the just become like Christ. This similarity is not evident through conduct, but stems from the new nature.