Are there divers ways of receiving this sacrament?
Yes, one can receive it spiritually or sacramentally (LXXX. 1).
What difference is there between these two ways?
This difference: those who receive the Holy Eucharist sacramentally only do not receive its effects; whereas those who receive it spiritually receive the effects thereof, whether this be by reason of the desire which moves them, or by reason of the actual reception of the sacrament which carries with it always the fulness of the sacrament's effect (LXXX. 1).
Can only man receive this sacrament spiritually?
Yes, because only man can believe in Jesus Christ, having the desire to receive Him as He is in this sacrament (LXXX. 2).
Can this sacrament be received sacramentally by sinners?
Yes, sinners who have the faith and who know what is the sacrament of the Holy Eucharist can receive it sacramentally whatever sins they may have on their conscience (LXXX. 3).
Does the sinner who receives this sacrament with the consciousness of his sins commit a sin in receiving this sacrament?
Yes; he commits a sacrilege, because by receiving this sacrament which contains Jesus Christ Himself, and which signifies the unity of Christ's mystic body which cannot exist except by faith and charity, he violates this sacrament, because without charity he endeavours to unite Jesus Christ to himself; and it is only charity which unites Jesus Christ to His members (LXXX. 4).
Is this sin especially grave?
Yes, because it insults the sacred humanity of Jesus Christ in the sacrament of His love (LXXX. 5).
Is this sin as grave as the external profanation of this sacrament?
No; for this latter sin implies the direct intention of insulting Jesus Christ in His sacrament, which is a sin of greater gravity (LXXX. 5, Obj. 3).
What is necessary in order to receive, as it behooves, this sacrament sacramentally?
First of all the use of reason, and the state of grace, and also the desire to gather the fruits of the spiritual life attaching to the reception of the sacrament (LXXX. 9, 10).
Next - The Catechism of the Summa - Tertia Pars - XXXIV. OF THE RECEPTION OF THE SACRAMENT OF THE HOLY EUCHARIST (B)