If has factual force here. It may be rendered by ‘because,’ ‘since,’ ‘as it is a fact that.’ Compare also such a rendering of the sentence as ‘God loved us much. Therefore let us love each other.’

So, that is, in the way shown by verses 9-10. Compare also a rendering of the clause like “if this is how God loved us” (Good News Translation).

We also ought to love one another: God’s love is his gift to us, but at the same time it is an obligation laid upon us. For ought compare also comments on 2.6.

Quoted with permission from Haas, C., de Jonge, M. and Swellengrebel, J.L. A Handbook on The First Letter of John. (UBS Handbook Series). New York: UBS, 1972. For this and other handbooks for translators see here .