The greatest commandment given by Jesus Christ himself is found in Matthew 22:37; love the Lord your God with all your heart, soul and mind. And the second greatest commandment is love your neighbor as you love yourself.
How do you love God with all your heart? By making Him your priority, by living for Him and by spending time with Him in prayer.
How do we love God with all our soul? By trusting in His plan for our lives even when we don’t understand our circumstances.
How do we love God with all our mind? By meditating on His word and by focusing our thoughts on His goodness, love and mercy.
How do we love our neighbor as ourselves? By showing them love and kindness and serving with a cheerful heart.