Objection 1. It seems that the effect of this sacrament is not hindered by venial sin. For Augustine (Tract. xxvi in Joan.), commenting on John 6:52, "If any man eat of this bread," etc., says: "Eat the heavenly bread spiritually; bring innocence to the altar; your sins, though they be daily, let them not be deadly." From this it is evident that venial sins, which are called daily sins, do not prevent spiritual eating. But they who eat spiritually, receive the effect of this sacrament. Therefore, venial sins do not hinder the effect of this sacrament.
Objection 2. Further, this sacrament is not less powerful than Baptism. But, as stated above (69, 9,10), only pretense checks the effect of Baptism, and venial sins do not belong to pretense; because according to Wisdom 1:5: "the Holy Spirit of discipline will flee from the deceitful," yet He is not put to flight by venial sins. Therefore neither do venial sins hinder the effect of this sacrament.
Objection 3. Further, nothing which is removed by the action of any cause, can hinder the effect of such cause. But venial sins are taken away by this sacrament. Therefore, they do not hinder its effect.
On the contrary, Damascene says (De Fide Orth. iv): "The fire of that desire which is within us, being kindled by the burning coal," i.e. this sacrament, "will consume our sins, and enlighten our hearts, so that we shall be inflamed and made godlike." But the fire of our desire or love is hindered by venial sins, which hinder the fervor of charity, as was shown in I-II, 81, 4; II-II, 24, 10. Therefore venial sins hinder the effect of this sacrament.
I answer that, Venial sins can be taken in two ways: first of all as past, secondly as in the act of being committed. Venial sins taken in the first way do not in any way hinder the effect of this sacrament. For it can come to pass that after many venial sins a man may approach devoutly to this sacrament and fully secure its effect. Considered in the second way venial sins do not utterly hinder the effect of this sacrament, but merely in part. For, it has been stated above (Article 1), that the effect of this sacrament is not only the obtaining of habitual grace or charity, but also a certain actual refreshment of spiritual sweetness: which is indeed hindered if anyone approach to this sacrament with mind distracted through venial sins; but the increase of habitual grace or of charity is not taken away.
Reply to Objection 1. He that approaches this sacrament with actual venial sin, eats spiritually indeed, in habit but not in act: and therefore he shares in the habitual effect of the sacrament, but not in its actual effect.
Reply to Objection 2. Baptism is not ordained, as this sacrament is, for the fervor of charity as its actual effect. Because Baptism is spiritual regeneration, through which the first perfection is acquired, which is a habit or form; but this sacrament is spiritual eating, which has actual delight.
Reply to Objection 3. This argument deals with past venial sins, which are taken away by this sacrament.
Source: Summa Theologica (New Advent)