Love of God and love of neighbour are one and the same love in grace. There are many ways that saints and lovers of God express this. There are as many particular ways to say a universal truth as there are particular people living and loving the Truth. For example, Jacques Maritain says,

Love of neighbour is the same love as love for God. Consequently, love for others unites us to God and makes us more like God.

And Saint John of the Cross says,

It is an evident truth that sympathy with neighbours increases the more, the more the soul is united with God through love. The more she loves God, the more she desires that he be loved and worshipped by all… tak[ing] trouble with great longing and heavenly desires and extraordinary carefulness to draw many to heaven with themselves.

(An evident truth!) And Saint Alphonsus says,

According to Saint John, love of God and love of our neighbour belong to one and the same commandment: “And this command we have from God, that he who loves God, loves also his brother” (1 Jn 4:21).

Saint Alphonsus roots the claim directly in the Apostles’ teaching. (No wonder the saints and lovers of God so often tell us that love of God and neighbour are one!)

There are many ways to express this truth. But perhaps one of the clearest and most insistent explanations, for me, lies with Saint Catherine of Siena and Saint Jane Frances de Chantal.

Saint Jane’s treatment is brief: God, she says, it the source of all good, but

No virtue is perfect that has not been tested by one’s neighbour.

Saint Catherine’s treatment is much lengthier and informs her entire thought. The theme occurs over and over and, each time it recurs, gathers momentum and additional layers of insight. Throughout her Dialogue, in which she asks questions of God the Father and gives the answers as she understands them, she and the Father often return to the theme of love of neighbour. Every virtue and every vice is manifested and proved in relation to our neighbours:

The number one is excluded, for, unless a man has a companion, I cannot be in the midst; this is no indifferent trifle, for he who is wrapped in self-love is solitary.

And,

I would have you know that just as every imperfection and perfection is acquired from Me, so is it manifested by means of the neighbour. And simple souls, who often love creatures with spiritual love, know this well, for, if they have received My love sincerely without any self-regarding considerations, they satisfy the thirst of their love for their neighbour equally sincerely.

Only by loving God can we love our neighbours. Only by going first to the One who created and redeemed us, can we love creatures “perfectly”.

Therefore to Me, in person, you cannot repay the love which I require of you, and I have placed you in the midst of your fellow, that you may do to them that which you cannot do to Me: that is to say, that you may love your neighbour of free grace, without expecting any return from him, and what you do to him, I count as done to Me…

Elsewhere:

This love you cannot repay to Me, but you can pay it to My rational creature, loving your neighbour without being loved by him in and without consideration of your own advantage, whether spiritual or temporal, but loving him solely for the praise and glory of My Name, because he has been loved by Me.

So that, it is in seeing the Trinity in our neighbours and acting on it, that we can love as freely as we ought; but in the first place, we receive this love from the Trinity, so that the only ones to whom we can give freely and gratuitously are our neighbours. We cannot repay God. But we can do to him in our neighbours. Every virtue and every vice acts in relation to our neighbours: who they are, what they are, where they come from, where they are going. Virtue comes from God. Love of God manifests itself fully and in bloom in love of neighbour.

It’s no secret that this is a mystery. But it’s a mystery of faith. Saint Catherine lays it out repeatedly in this form, but it is not a unique truth of hers. It’s simply the Gospel: “whatever you did to the least of these you did to me” (Mt 25:40). The implicit content of the Gospel made explicit – but the Gospel nonetheless.