venial and mortal sin

venial sin distinction brings an understanding to concept of "a righteous man"

This distinction between mortal and venial sin makes possible the truth of many passages in the Old Testament, such as “Noah was a righteous man, blameless in his time.” (Gen 6:9) This is how Job was blameless and upright. (Job 1:1,8; 2:3) This is how Joseph was a “righteous man.” (Mt. 1:19) This is how Abraham could have a discussion with God about the “righteous” and the wicked in Sodom; that conversation would not have been possible if all people are unrighteous. Does that mean that Noah never sinned? No, as Ecclesiastes 7:20 says, “there is not a righteous man on earth who continually does good and who never sins.” So Noah was both righteous and blameless, and yet not without sin. That is because though he sinned venially, he did not sin mortally. And that is true of all the Old Testament saints who died in friendship with God. They fulfilled the law not necessarily in the letter, but in the spirit of the law, which is the essence of the law. And the spirit of the law is agape. Because they had agape, they fulfilled the law, for as St. Paul teaches, agape fulfills the law (Rom 13:8, 10; Gal 5:14, James 2:8).

http://www.calledtocommunion.com/2011/11/why-john-calvin-did-not-recognize-the-distinction-between-mortal-and-venial-sin/  See this article for much more

Mortal sin/ 3 conditions

One act of mortal sin destroys charity (i.e. agape) in the heart:

Mortal sin destroys charity in the heart of man by a grave violation of God’s law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him. Venial sin allows charity to subsist, even though it offends and wounds it. (CCC 1855. )

According to the Catechism mortal sin destroys charity in the heart by a grave violation of God’s law. For a sin to be mortal, three conditions must together be met: “Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent.” (CCC 1857) Mortal sin turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to Him.

http://www.calledtocommunion.com/2011/11/why-john-calvin-did-not-recognize-the-distinction-between-mortal-and-venial-sin/

Mortal vs venial sin

More by Aquinus on mortal vs venial sin: 

. St. Thomas writes:

When the will sets itself upon something that is of its nature incompatible with the charity that orients man toward his ultimate end, then the sin is mortal by its very object … whether it contradicts the love of God, such as blasphemy or perjury, or the love of neighbor, such as homicide or adultery… . But when the sinner’s will is set upon something that of its nature involves a disorder, but is not opposed to the love of God and neighbor, such as thoughtless chatter or immoderate laughter and the like, such sins are venial. (Summa Theologica I-II Q.88 a.2.)⁴

http://www.calledtocommunion.com/2011/11/why-john-calvin-did-not-recognize-the-distinction-between-mortal-and-venial-sin/

 St Thomas Aquinas explains difference between mortal & venial sin

In the thirteenth century St. Thomas Aquinas explained the difference between mortal and venial sin, as follows:

Now the difference between venial and mortal sin is consequent to the diversity of that inordinateness which constitutes the notion [i.e. definition] of sin. For inordinateness is twofold, one that destroys the principle of order, and another which, without destroying the principle of order, implies inordinateness in the things which follow the principle: thus, in an animal’s body, the frame may be so out of order that the vital principle is destroyed; this is the inordinateness of death; while, on the other hand, saving the vital principle, there may be disorder in the bodily humors; and then there is sickness. Now the principle of the entire moral order is the last end, which stands in the same relation to matters of action, as the indemonstrable principle does to matters of speculation (Ethic. vii, 8). Therefore when the soul is so disordered by sin as to turn away from its last end, viz. God, to Whom it is united by charity, there is mortal sin; but when it is disordered without turning away from God, there is venial sin. For even as in the body, the disorder of death which results from the destruction of the principle of life, is irreparable according to nature, while the disorder of sickness can be repaired by reason of the vital principle being preserved, so it is in matters concerning the soul. Because, in speculative matters, it is impossible to convince one who errs in the principles, whereas one who errs, but retains the principles, can be brought back to the truth by means of the principles. Likewise in practical matters, he who, by sinning, turns away from his last end, if we consider the nature of his sin, falls irreparably, and therefore is said to sin mortally and to deserve eternal punishment: whereas when a man sins without turning away from God, by the very nature of his sin, his disorder can be repaired, because the principle of the order is not destroyed; wherefore he is said to sin venially, because, to wit, he does not sin so as to deserve to be punished eternally. (Summa Theologica I-II Q.72 a.5 co.)¹

[ a comment about  this on that blog: 

St. Thomas distinguishes between mortal and venial sin by explaining that mortal sin destroys the supernatural virtue of agape in the soul, and agape is the principle by which we are directed to heaven as our supernatural end. If agape is lost, the person is no longer ordered toward heaven, but instead toward some creature (e.g. himself) as his highest end. And he cannot be restored to friendship with God except by the power of God, sinceagape is supernatural, and we cannot give to ourselves what we do not have. Venial sins, by contrast, do not destroy agape from the soul, but are disordered in relation to the agape within the soul.² }

http://www.calledtocommunion.com/2011/11/why-john-calvin-did-not-recognize-the-distinction-between-mortal-and-venial-sin/

the following is from comment 257 here: http://www.calledtocommunion.com/2009/09/does-the-bible-teach-sola-fide/

When I say that God makes us righteous internally, I mean that He infuses into us sanctifying grace and agape. Sanctifying grace and agape are distinct in us, because our human essence is distinct from our will, which is a power of our soul. Sanctifying grace is that supernatural gift by which our nature is made to participate in the divine nature (2 Pet 1:4), and agape is that supernatural gift by which ourwill is granted to participate in the divine nature, by sharing in the supernatural love by which God loves Himself. In God, however, there is no such corresponding distinction between sanctifying grace and agape, because He is simple; His nature is Agape.

One of the weaknesses of our own time, due primarily to the success of modern science, is the tendency to attempt to quantify everything (and also overlook what cannot be quantified). Agape is something we either have or we do not. It is impossible to have some portion or part of agape, becauseagapehas no parts. This is why it is impossible to have 80% agape, or 30% agape, etc. A person either has agape or he does not. But the presence of agape within us does not entail that we cannot sin. So yes, you are one person, but you have a free will, and can freely choose between good and evil. The presence of agape within you in this present life does not remove from you the ability to choose evil. Sinning mortally, however, drives sanctifying grace and agape and the indwelling Trinity from the soul. Through repentance and the turning of the will back to God in contrition, in response to the work of the Holy Spirit, a person can receive again the gifts of sanctifying grace, agape, and the indwelling of the Trinity.

comment 98 here: http://www.calledtocommunion.com/2012/08/imputation-and-paradigms-a-reply-to-nicholas-batzig/#comment-40875

You wrote:

What about James 2:10?” For whoever keeps the whole law but fails in one point has become accountable for all of it.” Isn’t that implying a kind of list paradigm?

What makes this verse intelligible is the agape paradigm. Otherwise, how would it follow that a person who has coveted, has also murdered and committed adultery and not kept the Sabbbath, etc.? It wouldn’t follow. That’s James’ point two verses earlier, when he says,

“If, however, you are fulfilling the royal law, according to the Scripture, “You shall love your neighbor as yourself,” you are doing well.” (James 2:8)

From a Reformed point of view, James is speaking entirely hypothetically, since in that theological system, no one fulfills the royal law. But in the agape paradigm, the law is summed up in agape, such that he who is loving, is fulfilling the the whole law, and who does not love, is acting contrary to the principle underlying all the laws (James 2:10).