Objection 1. It would seem that venial sin can be taken away without mortal sin. For, on John 8:7: "He that is without sin among you, let him first cast a stone at her," a gloss says that "all those men were in a state of mortal sin: for venial offenses were forgiven them through the legal ceremonies." Therefore venial sin can be taken away without mortal sin.
Objection 2. Further, no infusion of grace is required for the remission of venial sin. but it is required for the forgiveness of mortal sin. Therefore venial sin can be taken away without mortal sin.
Objection 3. Further, a venial sin differs from a mortal sin more than from another venial sin. But one venial sin can be pardoned without another, as stated above (3, ad 2; 87, 3). Therefore a venial sin can be taken away without a mortal sin.
On the contrary, It is written (Matthew 5:26): "Amen I say to thee, thou shalt not go out from thence," viz., from the prison, into which a man is cast for mortal sin, "till thou repay the last farthing," by which venial sin is denoted. Therefore a venial sin is not forgiven without mortal sin.
I answer that, As stated above (Question 87, Article 3), there is no remission of any sin whatever except by the power of grace, because, as the Apostle declares (Romans 4:8), it is owing to God's grace that He does not impute sin to a man, which a gloss on that passage expounds as referring to venial sin. Now he that is in a state of mortal sin is without the grace of God. Therefore no venial sin is forgiven him.
Reply to Objection 1. Venial offenses, in the passage quoted, denote the irregularities or uncleannesses which men contracted in accordance with the Law.
Reply to Objection 2. Although no new infusion of habitual grace is requisite for the remission of venial sin, yet it is necessary to exercise some act of grace, which cannot be in one who is a subject of mortal sin.
Reply to Objection 3. Venial sin does not preclude every act of grace whereby all venial sins can be removed; whereas mortal sin excludes altogether the habit of grace, without which no sin, either mortal or venial, is remitted. Hence the comparison fails.
Source: Summa Theologica (New Advent)