Our capacity for mercy flows from our personal reception and realization of God's mercy provided in the Lord Jesus Christ.
"Be ye therefore merciful, as your Father also is merciful" (Luke 6:36).
To the degree we realize how patient and forgiving our Heavenly Father is to us, we will distribute the same to our offenders. Recall the servant in the Gospel of Matthew who had been forgiven a great debt, but who would not bestow the same mercy to one who owed him a pittance (Matthew 18:23-35). The servant had been forgiven what he owed. His realization of mercy, however, was faulty. He either quickly forgot or simply didn't appreciate the kindness of his creditor and the vastness of his absolved debt.
Born again believers in the Lord Jesus Christ require God's ongoing mercy as we live our Christian lives. In the absolute sense, our sins are forever and irrevocably forgiven. "Blessed is the man to whom the Lord will not impute sin" (Romans 4:8). Concerning our walk with God, however, acknowledging and confessing our transgressions is required for the maintaining of a clear conscious and ongoing communion with God. "If we walk in the light as He is in the light, we have fellowship one with another and the blood of Jesus Christ cleanseth us from all sin" (I John 1:7). Remembering that God delights in mercy, and responding by humble and sincere confession of sin, maintains the awareness of our personal reception of His abundant pardon. Upon this basis, we then find ourselves able to bestow mercy upon others as the pardon we consciously receive focuses our attention far more upon our own sins than those of others.
The debt owed by other people to us pales in comparison to the sins for which God has forgiven us. Our Heavenly Father commands that we forgive as we have been forgiven, and He enables us by His indwelling Spirit to "be ye therefore merciful as your Father in heaven is merciful." Much remembrance of His mercy personally received will go far in much determination and enabling to bestow mercy upon others. The Lord Jesus is revealed thereby, our hearts are kept in peace, and the Gospel is proclaimed in a spiritually tangible expression that leads others to our God of mercy.
"Freely ye have received, freely give."