Why is it that a mortal sin will end your relationship with God? It would seem that God becomes easily offended if one’s sin leads Him to turn away—to take his divine ball and go home, so to speak. Often times that is how many portray the concept of mortal sin. A person after reflection decides to commit a serious sin with his full will. God, it appears, gets angry and then decides to take away his grace until such time the relationship is restored, if at all.
However, this is not correct. The real understanding is more something we can see in circuit design, whether a water circuit or an electronic circuit. We will take the water circuit as the best illustration.
Let’s imagine a water circuit has two inputs and one output. A valve allows only one input to the one output at a time. So if you have water from the well as one source and water from the town as the other, you would put in such a valve to choose between the two sources when necessary. When you were using well water, you would turn the valve to exclude the city water and vice versa. The valve prevents you from using both sources at the same time.
It is the same with mortal sin. You have the source that is God’s grace always flowing and you have the source of pleasure from a mortal sin which you prevented from flowing by choosing not to sin.
In order to commit sin, you, with full knowledge of its gravity, reject God’s grace to do what you prefer in the pleasure derived from the sin: Sex out of marriage, sexuality not open to life, drug abuse, stealing, refusing to forgive, engaging in the occult, choosing to skip Mass on Sundays, etc. You, in a sense, have to move your valve from that which inputs God’s ever flowing grace, to one that inputs the pleasure from sin. So, you cut off the source of God’s grace in order to dive into this sin. You cannot have both sources open. So it is not God who gets offended and turns from you, it is you who cut off God’s grace by choosing to close yourself to that input and open to something other than Him. You cannot have both sources at the same time.
You have to choose one or the other. So it is not God who takes his ball and goes home, it is you who change the valve so that your will be done which closes off the source of your life which is God’s grace that His will be done. God's grace is still there and even overflowing, but you have any input for this grace cut off.
This is why by the way, you cannot accidentally commit a mortal sin, it is as deliberate an act as changing the source on a valve in your home between well water and town water.
Others will tell you that it actually does not matter because God’s grace is so strong that it will overpower the valve and even the negative force of sin. Nothing in Magisterial teaching, nothing from the saints or the Catechism teaches this—Nothing! People may teach this today, but they are not speaking accurate Catholic teaching. One cannot choose sin and God’s grace at the same time. In fact, the Bible warns against this intensely.
Let’s change our plumbing system to one that chooses between hot and cold water and can input both simultaneously: What happens when you choose to turn on the hot water and the cold water at the same time. You get lukewarm water. Now check out Revelation 3:16, that is the one position that God rejects, the mixing of two opposing sources that cheapen both.
So, the key to the understanding the teaching of mortal sin is that it is not some arbitrary action on the part of God, but is in fact, a purposeful act on our part to reject God’s grace for some temporal pleasure and that is never a good thing.
This is the point of confession, through the Sacrament of Reconciliation, one repents of his or her sin and receives absolution, this changes the “valve” position back to exclusively being a source of God’s grace and to lead us closer to Him and to all He is creating us to be.