The common way of looking at salvation is to view it as a matter of what we have done, and what has been forgiven. If we commit a sin, we are guilty and go to Hell. If we confess, we have it wiped off our record and go to Heaven. This view works for a general understanding and everyday conversation, but it falls short when it comes to what is actually happening.
We are born in original sin. This means we are without God’s Grace. When we get baptized, we receive God’s Grace. Baptism puts us in a State of being in and with God’s Grace. We call this being in a state of Grace. We don’t go to Hell or Heaven based on what we have done, but based on whether we are in a state of Grace or not. Because of our sin, we can never be fit for Heaven. Even being forgiven in the way people usually understand it wouldn’t do us any good. You see, having a “clean slate,” as we usually think of it, doesn’t matter because we are by our very nature corrupt and impure, and as we know nothing impure can enter into Heaven. The common way of looking at Baptism is that it “washes away” original sin. In one sense this is true, because it alone Christ makes us able to receive God’s Grace and be in a State of Grace. However, even Baptism leaves behind an impurity which we call concupiscence. Concupiscence is a “disordered nature,” a nature which is not natural and not according to God’s design for us. In “laymen’s” terms, it is an attachment to sin: something that makes us “want” to sin. Concupiscence is an impurity. Because of that, even being forgiven doesn’t really cut it if we want to get to Heaven.
ONLY through God’s Grace may we make it to Heaven. God’s Grace gives us that free gift of Salvation that nothing can earn. When we are God’s Grace, He promises to bring us to Heaven even though we don’t deserve it. This is truly Grace indeed! Therefore, through God’s Grace, he gives to us Purgatory: a purification to remove the impurities in our souls, such as concupiscence.
Sin also has two ways of looking at it: the common way and the technical way. The common way to look at sin is to look at it as breaking some sort of law or rule, like “do not commit adultery.” The technical way to view sin is as a rejection of God’s Grace. The original sin of Adam was something very simple: rejecting God and going against Him. This is what all sin is in its essence. Killing is not a sin because it is intrinsically wrong, but because it is a rejection of God’s commandment and therefore truly a rejection of God Himself (which is essentially why God Himself can take lives without sinning). God can’t sin not because He is so powerful and righteous (though He is), but because sin is nothing more or less than the rejection of God. Put another way, a sin is something which falls short of loving God.
Venial sins are those acts which don’t completely reject God. They are “minor” things which fall short of loving God, but don’t actually reject Him. A mortal sin is a sin which at its core rejects God. This is why a mortal sin has its three qualifications: grave matter, full knowledge, and full will. To reject God, a person must know it is God he is rejecting, and also must have the want. This is why for a Christian, only mortal sin can send a person to Hell. When we are in God’s Grace, we will be purified and goto Heaven. Only rejecting this Grace and becoming Graceless will condemn us because if we don’t reject God’s Grace it will save us. An unbaptized person never was in a state of Grace in the first place, so it doesn’t matter if they have full knowledge or will when they commit sin. They may not go to Hell, but without the Grace to be purified they can’t make it to Heaven (When they Church talks about people outside its visible boundaries being saved, it is assumed that they have received a Baptism of Desire).
A mortal sin, therefore, is an act which completely and utterly rejects God and therefore His Grace which alone would save a person. A venial sin is an act which is not completely in line with loving God but doesn’t reject Him. Think of parents and a teenager. If a teenager lies and tells his parents that he did his homework when he didn’t, this is not completely living up to the idea of loving them, but it certainly doesn’t reject them. If the teenager sneaks out in the middle of the night, he is completely missing the goal of loving them and he rejects them.
When we completely miss the goal of loving God, this is mortal sin. Therefore, when a person denies God exists, this completely misses the goal of loving God. It is a rejection of Him. When a person kills, it completely falls short of loving God. When a person tells a white lie, or steals a dollar, it doesn’t completely fall short of loving God, but it doesn’t completely live up to it. This is a venial sin.