In the gospel of John the Lord says: In this will all men know that you are my disciples, if you have love for each other. In a letter of the same apostle we read: Beloved, let us love one another, for love is from God, and everyone who loves is born of God and knows God; he who does not love does not know God, for God is love.
Notice the Christian anthropology that is at work here. St. Leo does not presume that man is an individual substance of a rational nature as the Greeks understood him. Rather, man is an image of the divine Person of the Son who, as Son, is pure relation to the Father. Man, therefore, is a finite person whose destiny is to become son as the Son, i.e. to be totally “for” the Father, and therefore to be totally at the service of everyone else. He achieves his maturity by transcending himself for others. Therefore, St. Leo continues:
“The faithful should therefore enter into themselves and make a true judgment on their attitudes of mind and
heart. If they find
some store of love's fruit in their hearts, they must not doubt God's presence
That is, if they experience love for others in themselves, they are experiencing the imaging of the divine Son, and therefore the presence of the Son in them.
“If they would increase their capacity to receive so great a guest, they should practice greater generosity in doing good, with persevering charity.”
Therefore, the more they give, the more they are imaging, and therefore, the more they are.
“If God is love, charity should know no limit, for God cannot be confined. Any time is the right time for works of charity, but these days of Lent provide a special encouragement. Those who want to be present at the Lord's Passover in holiness of mind and body should seek above all to win this grace, for charity contains all other virtues and covers a multitude of sins.
“As we prepare to celebrate that greatest of all mysteries, by which the blood of Jesus Christ did away with our sins, let us first of all make ready the sacrificial offerings of works of mercy. In this way we shall give to those who have sinned against us what God in his goodness has already given to us.
“Let us now extend to the poor and those afflicted in different ways a more open-handed generosity, so that God may be thanked through many voices and the relief of the needy supported by our fasting. No act of devotion on the part of the faithful gives God more pleasure than that which is lavished on his poor. Where he finds charity with its loving concern, there he recognizes the reflection of his own fatherly care.
“In these acts of giving do not fear a lack of means. A generous spirit is itself great wealth.”
This means that the more you give away, the more you are. And then the explicit affirmation of the Christological anthropology that undergirds his thinking: It is Christ Who is feeding, and Christ Who is being fed:
“There can be no shortage of material for generosity where it is Christ who feeds and Christ who is fed. In all this activity there is present the hand of him who multiplies the bread by breaking it, and increases it by giving it away.
“The giver of alms should be free from anxiety and full of joy. His gain will be greatest when he keeps back least for himself. The holy apostle Paul tells us: He who provides seed for the sower will also provide bread for eating; he will provide you with more seed, and will increase the amount of your goodness, in Christ Jesus our Lord, who lives and reigns with the Father and the Holy Spirit for ever and ever. Amen.
Note that Leo is not talking about “Charity” as an “accident” that inheres in an already ontological established substance as “thing-in-itself.” It is an “attitude” as an orientation of the person as subject (”I”) by which the whole self is given to the other, as the Self of the Son is given to the Father “for” us.
Consider Ratzinger’s theological description of the Son, not as “substance” (being-in self) but as “nothing” in self:
“The Son as Son, and in so far as he is Son, does not proceed in any way from himself and so is completely one with the Father; since he is nothing beside him, claims no special position of his own, confronts the Father with nothing belonging only to him, retains no room for his own individuality, therefore he is completely equal to the Father. The logic is compelling: if there is nothing in which he is just he, no kind of fenced-off private ground, then he coincides with the Father, is ‘one’ with him. It is precisely this totality of interplay that the word ‘Son’ aims at expressing. To John ‘Son’ means being-from-another; thus with this word he defines the being of this man as being from another and for others, as a being that is completely open on both sides, knows no reserved area of the mere ‘I.’ When it thus becomes clear that the being of Jesus as Christ is a completely open being, a being ‘from’ and ‘towards,’ that nowhere clings to itself and nowhere stands on its own, then it is also clear at the same time that this being is pure relation (not substantiality) and, as pure relation, pure unity. This fundamental statement about Christ becomes, as we have seen, at the same time the explanation of Christian existence. To John, being a Christian means being like the Son, becoming a son; that is, not standing on one’s own and in oneself, but living completely open in the ‘from’ and ‘towards.’ In so far as the Christian is a ‘Christian,’ this is true of him. And certainly such utterances will make him aware to how small an extent he is a Christian./
 J. Ratzinger, “Introduction to Christianity,” Ignatius (1990) 134.
 Ratzinger: “I think it is not unimportant to note how the doctrine of the Trinity here passes over into a statement about existence, how the assertion that relation is at the same time pure unity becomes transparently clear to us. It is the nature of the Trinitarian personality to be pure relation and so the most absolute unity. That there is no contradiction in this is probably now perceptible. And one can understand from now on more clearly than before that it is not the ‘atom,’ the indivisible smallest piece of matter, that possesses the highest unity; that on the contrary pure oneness can only occur in the spirit and embraces the relativity of love. Thus the profession of faith in the oneness of God is just as radical as in any other monotheistic religion; indeed only in Christianity does it reach its full stature. But it is the nature of Christian existence to receive and to live life as relatedness, and thus to enter into that unity which is the ground of all reality and sustains it. This will perhaps make it clear how the doctrine of the Trinity, when properly understood, can become the nodal point of theology and of Christian thought in general.”