For St. Thomas Aquinas, “presence” is a key element in friendship. Thomas relates the Incarnation itself to Jesus’ desire to be with us: “…this belongs to Christ’s love, out of which for our salvation, He assumed a true body of our nature… because it is the special feature of friendship to live together with friends…” (3a. 75, 1).
Oftentimes, we think of religion as our obligations to God. Thomas opens up another approach, Jesus desire to be with us, even as a friend.
The Eucharist is the concrete presence of Christ with us, “The presence of Christ’s true body and blood.” This is not a symbolic presence but, rather, the Eucharist “contains Christ Himself crucified, not merely a signification or figure, but also in very truth” (3a 75, 1). We perceive Christ’s presence not by the senses or by understanding but by “faith alone“.
Does it make a difference that Jesus is really present in the Eucharist and not in some spiritual sense? It is like the difference between a friend being personally present and a friend sending an e-mail or texting a message.
We hope to be in the presence of Jesus eternally: “He promises His bodily presence as a reward…” (3a. 75, 1). Eternal life is being with Jesus.
His presence with us now prepares us for His future presence: “Yet meanwhile in our pilgrimage He does not deprive us of His bodily presence but unites us with Himself in this sacrament through the truth of His body and blood. Hence, He says: ‘Whoever eats My flesh and drinks My blood, abides in Me and I in him’ (Jn 6:57).”
Because of Christ’s desire for friendship with us in the Sacrament, this is the sacrament of charity: “Hence this sacrament is the sign of supreme charity and lifts up our hope, from such familiar union of Christ with us” (3a. 75, 1).
We are united not only with Christ the Person but we are united with the members of Christ’s Body: “The thing signified is the unity of the mystical body of Christ which is an absolute required for salvation” (3a. 73, 3). The Eucharist binds us with our brothers and sisters in Christ.
The Eucharist itself is the source of the charity: it is “the sacrament of charity because it symbolizes charity and brings it about” (3a, 78, 3). The Eucharist gives us the charity that we need.
As Christ lives in us, we are transformed by His presence. Christ doesn’t just come and go without any effect on us: “The particular effect of this sacrament is the change of the person into Christ, as the Apostle said: ‘I live, yet not I, Christ truly lives in me’ (Gal. 2:20). ” Thomas explains: “There is a difference between bodily food and spiritual food. Bodily food is changed into the substance of the person who eats it...but spiritual food changes man into itself” (3a, 73, 3).
Because the Eucharist contains Christ’s Body, all the other sacraments are directed to it: “The Eucharist is the summit of the spiritual life and all the sacraments are ordered to it... They sanctify us and prepare us to receive the Eucharist or to consecrate it” (3a. 73, 3).
In this way, the Eucharist perfects the other sacraments: “This sacrament, which contains Christ Himself… is perfective of all the other sacraments…” (3a. 75, 1).
The Eucharist is the sacrament of the Passion of Christ because in it a man is brought to spiritual perfection in being closely united with Christ who died for us. Hence, just as Baptism is called the ‘sacrament of faith,’ which is the beginning of spiritual life, so the Eucharist is called the ‘sacrament of charity,’ which is the bond of perfectness, as we read in Colossians (3a. 73, 3 ad 3).
Denis Vincent Wiseman, O.P.
 Third part of the Summa Theologiae, question 75, article 1.