God’s love has been poured into our hearts through the Holy Spirit who has been given to us. Romans 5:5
“The Comforter gives a sweet and plentiful evidence and persuasion of the love of God to us, such as the soul is taken, delighted, satiated withal. This is his work, and he doth it effectually. To give a poor sinful soul a comfortable persuasion, affecting it throughout, in all its faculties and affections, that God in Jesus Christ loves him, delights in him, is well pleased with him, hath thoughts of tenderness and kindness towards him; to give, I say, a soul an overflowing sense hereof, is an inexpressible mercy.”
John Owen, Works (Edinburgh, 1979), II:240.