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To do this as we ought, the first thing required is freedom from mortal sin. The Apostle strongly said that whoever eats the bread or drinks the chalice of the Lord unworthily is guilty of the body and blood of the Lord. In other words, those who approach Communion in the state of mortal sin commits a sin comparable to the sin of those who crucified Christ, because both are sins against the body and blood of Jesus, although in different ways.
Moreover, what can follow from the union of two different things such as Christ and a sinner, but the corruption of the sinner? Things that are alike are easily joined together, such as water and wine; but things that are contrary, such as fire and water, can never be mixed without the destruction of one. How can the good be mixed with the bad, the humble with the proud, the pure with the unclean? Hence, there must be some kind of likeness effected between Christ and the Christian if the latter is to be united worthily with Him."
(Ven. Louis of Granada, O.P. from Summa of the Christian Life)