The Catholic Church affirms that all baptised persons who believe in Christ are in some way joined to her (cf. Unitatis Redintegratio, §3). However, the Eucharist is not merely a symbol of unity—it is its culmination and visible expression of union and communion in Christ. The Church’s discipline of “closed communion” is not a rejection of others’ faith, but a safeguard of the sacrament’s integrity. As the Catechism of the Catholic Church teaches, “Ecclesial communities derived from the Reformation… have not preserved the proper reality of the Eucharistic mystery in its fullness” (CCC §1400). Because the Eucharist signifies full communion in faith, sacraments, and ecclesial governance, it cannot be shared where that unity does not yet exist (Redemptionis Sacramentum, §84).
You rightly cite Matthew 18:20, and Christ is present where believers gather in His name. Yet His presence in the Eucharist is of a different order—“substantial,” “real,” and “true” (CCC §1374)—requiring full communion with the Church He founded. The Catholic Church earnestly desires unity, but this unity must be rooted in truth and sacramental fidelity. As Pope Benedict XVI affirmed, “The Eucharist is intrinsically linked to full communion with the Church” (Sacramentum Caritatis, §56). Your love for your Catholic brethren is appreciated, and the Church reciprocates that love while remaining faithful to the deposit of faith entrusted to her by Christ.