16 Having reflected on the nature of love and its meaning in biblical faith, we are left with two questions concerning our own attitude: can we love God without seeing him? And can love be commanded? Against the double commandment of love these questions raise a double objection. No one has ever seen God, so how could we love him? Moreover, love cannot be commanded; it is ultimately a feeling that is either there or not, nor can it be produced by the will. Scripture seems to reinforce the first objection when it states: “If anyone says, ‘I love God,' and hates his brother, he is a liar; for he who does not love his brother whom he has seen, cannot love God whom he has not seen” ( 1 Jn 4:20). But this text hardly excludes the love of God as something impossible. On the contrary, the whole context of the passage quoted from the First Letter of John shows that such love is explicitly demanded. The unbreakable bond between love of God and love of neighbour is emphasized. One is so closely connected to the other that to say that we love God becomes a lie if we are closed to our neighbour or hate him altogether. Saint John's words should rather be interpreted to mean that love of neighbour is a path that leads to the encounter with God, and that closing our eyes to our neighbour also blinds us to God.
Source: Deus Caritas Est (Vatican.va)