Summary: Love originates from God, and those who love know Him, because God is love itself. God demonstrated His love by sending His Son as a sacrifice for our sins.
Beloved, let us love one another, because love is of God; everyone who loves is begotten by God and knows God.
Whoever is without love does not know God, for God is love.
In this way the love of God was revealed to us: God sent his only Son into the world so that we might have life through him.
In this is love: not that we have loved God, but that he loved us and sent his Son as expiation for our sins.