Draw near with a true heart: you that carry the vessels of the Lord

"How necessary it is to prepare beforehand to receive Holy Communion worthily...

The first condition necessary for a worthy Communion is a lively faith. Faith is the groundwork and foundation of every virtue and of every meritorious action. Hence St Paul says, Let us draw near with a true heart in fullness of faith. Excite, therefore, a lively faith in your soul, as often as you frequent this Holy Sacrament; reject every temptation arising from the dictates of the senses against a firm belief in the mystery; tell them that the veracity of God Himself, whose revelations are not to be judged at the tribunal of man's weak senses and limited understanding, is the pledge of its truthfulness.

The second preparation requisite for a worthy Communion is purity of mind and conscience. In proportion to the perfection of this purity you will receive the benefits of the Sacrament. Hence Isaias exclaims, Be ye clean, you that carry the vessels of the Lord. The Church recommends the same purity to us; hence she has ordained that the Eucharist shall be consecrated on clean corporals. These corporals, says the Angelic Doctor, are first washed, then wrung, and lastly dried: so he who approaches the Sacrament must first wash away his sins by the tears of penance; he must then drain from his soul by mortification every affection to sin that may linger in it; thirdly, the fire of charity must expel from it every predisposition to relapse. Examine if these be your dispositions when you approach the Communion rail."

--Eternal Thoughts from Christ the Teacher Vol. II by Richard Cardinal Cushing