The greatest commandment and the mark of a disciple.
God is love. We love because He first loved us, and we are called to love God with all our heart and our neighbour as ourselves.
What the Scriptures say
He that loveth not knoweth not God; for God is love. 1 John 4:8 · KJV
A new commandment I give unto you, That ye love one another; as I have loved you, that ye also love one another. By this shall all men know that ye are my disciples, if ye have love one to another. John 13:34-35 · KJV
Charity suffereth long, and is kind; charity envieth not; charity vaunteth not itself, is not puffed up, Doth not behave itself unseemly, seeketh not her own, is not easily provoked, thinketh no evil; Rejoiceth not in iniquity, but rejoiceth in the truth; Beareth all things, believeth all things, hopeth all things, endureth all things. 1 Corinthians 13:4-7 · KJV
Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. This is the first and great commandment. And the second is like unto it, Thou shalt love thy neighbour as thyself. Matthew 22:37-39 · KJV