St. Thomas Aquinas
“The sin of unbelief, which fundamentally severs a man from the unity of the Church, simply speaking, makes him to be utterly unfit for receiving this sacrament; because it is the sacrament of the Church’s unity, as stated above. Hence the unbeliever who receives this sacrament sins more grievously than the believer who is in sin; and shows greater contempt towards Christ Who is in the sacrament, especially if he does not believe Christ to be truly in this sacrament; because, so far as lies in him, he lessens the holiness of the sacrament, and the power of Christ acting in it, and this is to despise the sacrament in itself.”
Source: St. Thomas Aquinas, Summa Theologica