The short answer
Love is the form of Christian life. We do not earn grace by loving; rather, grace makes supernatural love possible and love reveals living faith.
Jesus said
Jesus places love at the center of the commandments and identifies merciful action toward others with service to Him.
Matthew 22:37–40John 13:34–35Matthew 25:31–46
The apostles taught
Paul teaches that faith works through love, while James insists that faith without works is dead.
Galatians 5:61 Corinthians 13:1–13James 2:14–26
The Catholic synthesis
Charity is the greatest theological virtue. It unites us to God and animates every authentic Christian act.