It's commonly known that Evangelical Christians generally believe that all sins are the same to God. That is, equally offensive and equally transgressive. This concept was articulated in drastic way during a local Christian radio show I was listening to back around Christmas. The host illustrated this idea by saying that murder and staying home from church on Sunday for no good reason are equal sins in the eyes of God because all sins are the same to God.
Of course in human society, this is not the case (e.g., no death penalty for parking violations). Even in the same crime, you have various degrees of offense (like first degree murder, second degree murder, and manslaughter). Neither is it the case that all sins are viewed as equal in traditional Christian theology--both Catholic and Protestant.
In Catholic theology (including Anglicanism), there are two types of sin called "mortal" and "venial." This distinction comes from 1 John 5:16-17, which reads: "If any one sees his brother committing what is not a mortal sin, he will ask, and God will give him life for those whose sin is not mortal. There is sin which is mortal; I do not say that one is to pray for that.
All wrongdoing is sin, but there is sin which is not mortal."
This had been interpreted to mean that prayer and penance is efficacious for venial sins, but for mortal sins, one has to be sacramentally absolved and restored to life. Venial sins are in the nature of "falling short of the glory of God" while mortal sins are deliberate transgressions which reject our fellowship with the source of life, thereby leading to spiritual death. Just as in human relationships, there are some wrongdoings that are imperfections and there are others that are relationship ending. Within these categories, the consequences and punishments due to sins vary by degree and circumstance and are reserved to the judgment of God.
What many people do no know is that this idea of all sins being equal before God is not inherent to Protestantism either. The scripture usually cited to support the idea of all sins being equal is James 2:10, which reads: "For whoever keeps the whole law but fails in one point has become guilty of all of it." So how does the great Protestant reformer John Calvin interpret this verse? The same as a Catholic and all the other Protestant reformers of his day would--that sins are not equal before God.
In his Commentary on the Book of James, Calvin writes: "What alone he means is, that God will not be honored with exceptions, nor will he allow us to cut off from his law what is less pleasing to us. At the first view, this sentence seems hard to some, as though the apostle countenanced the paradox of the Stoics, which makes all sins equal, and as though he asserted that he who offends in one thing ought to be punished equally with him whose whole life has been sinful and wicked. But it is evident from the context that no such thing entered into his mind.
"For we must always observe the reason anything is said. He denies that our neighbors are loved when a part only of them is through ambition chosen, and the rest neglected. This he proves, because it is no obedience to God, when it is not rendered equally according to his command. Then as the rule of God is plain and complete or perfect, so we ought to regard completeness; so that none of us should presumptuously separate what he has joined together. Let there be, therefore, a uniformity, if we desire rightly to obey God. As, for instance, were a judge to punish ten thefts, and leave one man unpunished, he would betray the obliquity of his mind, for he would thus shew himself indignant against men rather than against crimes; because what he condemns in one he absolves in another.
We now, then, understand the design of James, that is, that if we cut off from God’s law what is less agreeable to us, though in other parts we may be obedient, yet we be come guilty of all, because in one particular thing we violate the whole law. And though he accommodates what is said to the subject in hand, it is yet taken from a general principle, — that God has prescribed to us a rule of life, which it is not lawful for us to mutilate. For it is not said of a part of the law, 'This is the way, walk ye in it;' nor does the law promise a reward except to universal obedience."
To summarize, Calvin attributes the idea that all sins are the same to the Stoics, not St. James, and adds that "it is evident that no such thing entered into his mind." So how did this idea come into Evangelical Christian theology?
I have not been able to determine when the idea came into Christian thinking, though it is most likely within the last century or two. But it is clear where this idea comes from and how this misunderstanding came into Evangelical theology. It appears that this is a corruption of the idea of Torah and covenant applying to the whole community and to the whole Torah.
The Old Testament, and Deuteronomy in particular, makes clear that the terms of the covenant are to abide by the Torah as a whole; there are not covenants for each commandments. Thus, to violate one commandment is to put oneself out of covenant relationship with God. To violate one is to bring about the curse of being out of covenant just as much as violating them all. "Keep the whole commandment that I command you this day . . . Cursed be anyone who does not confirm the words of this law by doing them" (Deut 27:1b,26). Paul confirms this idea as a tenant of Judaism when he notes, "I testify again to every man who receives circumcision that he is bound to keep the whole law" (Gal 5:3).
Just as the body of commandments was to be kept as one, so also everyone in the community were to keep the Torah in its entirety (thus the OT emphasis on purging transgressors from the community). These were the terms of the Mosaic covenant. It's no wonder that Paul asserts that it cannot be done, therefore Christ has kept the law on our behalf. And in fact the Pentateuch seems to acknowledge that it won't happen, but they are still to do their best and God's mercy would take care of the rest.
However, none of this implies that each sin is just as serious as another, but only that breaking any commandment violates the terms of the covenant. In fact, just the opposite is indicated in that there are different punishments proscribed for different transgressions (e.g., no death penalty for parking violations) and different means of atonement for different transgressions.
At some point, it appears that out of an anti-Catholic polemic, this idea of all sins violating covenant was taken up to mean all sins are the same before God and thus was used to refute and undermine the sacramental and penitential system of the church. The idea that all sins are the same is also used to reinforce the doctrine of eternal security ("once saved, always saved"), although it is interesting that Calvin himself never made that leap.