Sinning Chritian's moral code as "irrational" as an atheists code

[Trantor]

For the sake of concretness consider a Christian who believes that there is an Absolute Morality as defined by God and revealed to Man in whatever way the particular sect of his/her faith believes..the literal word of God as expressed in the Bible, a variation on it, whatever.

Consider also an atheist who believes there is no Absolute Moral code.

It has been argued that the atheist's morality is irrational because the code followed is not an absolute code and because it is irrational is worthless and incomprehension expressed that any moral code is followed by atheists consequentially. ("Why not just go out and rape women" argument)

Now consider the Chritian. Humans sin, in their belief, because that is the nature of Man. No one is perfect but they believe one should aspire to follow the absolute Moral code of Christianity. But human are weak, frail etc and do sin.

Consider then the sinning Christian. Aware of the sin, knowing they sin. They will not disobey all the moral laws of their religion but will sin nonetheless..some big some small. Assume all are truly sorry and confess, repent whatever. I am not talking about hypocrosy, just human fallibilty.

While being fallible, the Christian will, by assumption, be not be following the Absolute code they believe in but will be breaking some laws. How do they decide which ones to break: there is no absolute moral code on what moral laws can be broken.

So the ACTUAL moral code of Christian sinners is not the Absolute code. They somehow pick out a subset of laws they are prepared to follow(albeit reluctantly, perhaps). The set of non-absolute laws followed by the inevitably-sinning Christian has no more a basis in God than the set of non-absolute laws followed adopted by the atheist.

Why, then, do the irrational, no meaning arguments not apply to sinning Christians?