There is a double reading of these words according to the original.
1. They may be read, let us love him because he first loved us, by way of motive, denoting, that believers have great reason to love God with their choicest and highest affections, forasmuch as he has loved them, and first loved them.
2. They are here read by way of casuality, we do love him, because he first loved us; intimating, that God's love to us is the root and spring of our love to him, and to one another: all our love to saints is the effect of his preventing love to us, and but a reflection of those beams of love which God has first cast upon us; if God's love to us had been a mere dependent consequence of our love to him, how uncertain should we be of its continuance? But his love to us was the antecedent cause of our love to him; we therefore love him, because he first loved us.