To answer this question, I think we must first explicitly lay out the requirements of the Church for a sin to be mortal:
- The sin must be of a grave matter.
- The sin must be committed with full knowledge and awareness of the sinful action and the gravity of the offense.
- The sinful action must be committed with deliberate and complete consent.
With respect to these three points, any sin can then become venial if any of them are not fully applicable:
- The sin is not of a grave matter.
- The sinner does fully understand that the action is sinful or does not know that the matter of the sin is grave.
- The sinner does not fully consent to the sin or is under coercion (whether through outside pressure or mental illness).
Now, if the sinner lives their life in habitual sin, but constantly falls under the exception of numbers 2 and 3, then I do not believe that these venial sins become mortal, for they simply do not fully understand their action or are under an influence not fully under their control. If the sinner habitually commits venial sin simply because it is technically not a grave matter, so it is alright to do so, then I believe that this can and does become mortal after a period of time.
We must remember, Christ rebuked the Pharisees for following the letter of the Law without adhering to the Spirit which drove it. “Woe to you, scribes and Pharisees, you hypocrites. You pay tithes of mint and dill and cummin, and have neglected the weightier things of the law: judgment and mercy and fidelity. [But] these you should have done, without neglecting the others.” (Mt. 23:23) I believe that this habitual flaunting of legalities drives to a deeper sin. Fidelity to the Holy Spirit which drives the Law (in this case the technical requirements for mortal sin found in the Catechism) is, in itself, a grave matter.
I believe that the citation of Galatians 6:7 in the original post is quite relevant. “Make no mistake: God is not mocked, for a person will reap only what he sows.” Those who refuse to sow fidelity to the Spirit of the Gospel, reap nothing but the death of mortal sin.
This does not, however, give us authority to judge those who do live in habitual venial sin. That power alone resides with our Lord. Instead, we must turn to fraternal correction to build up our brothers and sisters in this state so that they can freely make the choice to turn away from their sin.
God Bless,
Br. Ben, CRM