Humanity acquires knowledge over time that replaces old ideas and reveals their impotence. We have come a long way since the ancient immorality of the Old Testament. We no longer murder children for cursing their parents. We no longer murder a person for working on the Sabbath, and the only time we slay entire cities, sparing none, for following a different God, is when we do it for religious reasons. All of these behaviors are not simply condoned but demanded in several books of the Old Testament with no exceptions (Deuteronomy and Leviticus specifically). We have done away with slavery today, and most people see the act as one of the most egregious offenses against human dignity. The Old Testament condones slavery - it even condones beating slaves (with stipulations) - and Jesus clearly took no issue with slavery. Many Biblical demands are immoral, and if we wish to behave ethically, we must disobey them.
It is at this point that Christians may begin to interject in defense of their faith, so I will deal with two likely objections for a moment.
The first foreseeable objection is that Christians rely primarily on the New Testament for ethics. The New Testament speaks mostly of loving thy neighbor, displaying meekness, and accepting subordination and servitude before all others. To hold a Christian to the standards of the Old Testament is to misrepresent the Christian. Following in Jesus' footsteps is surely ethical.
(Whether subordinating one's self before his enemy is or is not ethical requires a long digression - but it is a debate which I will gladly take part in if anyone wishes it.)
My response to this objection is fourfold. First, notice that this objection does not deal with the central issue of the initial argument, which is that the Old Testament allows no lenience in cherry-picking which laws to follow. If Christianity could exist without the Old Testament, then my initial argument would be flawed. It cannot. The entire reason that Jesus is believed to be the son of God is precisely because he fulfilled prophecies laid out in the Old Testament.
Another reason that Christian ethics cannot be divorced from the Old Testament is because Christianity relies solely on the Old Testament for its chief code of ethical law - the Ten Commandments.
A third reason that the Old and New Testament laws are inextricably linked is that Jesus said specifically that each and every word of the Old Testament laws must be followed, without changing a "jot"
A fourth reason is the concept of the Trinity. The Father, the Son, and the Holy Spirit, though appearing to be wholly distinct entities, are actually one in the same.
The second objection I foresee is that my argument attempts to judge God by human standards. We are myopic and small, and our limited view of good and evil just doesn't cut it when we speak of ethics on the Godly level. God is perfect - infinitely loving and infinitely just. If and only if God exists is there an objective set of ethics. God does exist, and so his ethical code is objective and immune to criticism.
For the purpose of responding to this argument directly, we will assume that God certainly exists. There are really two pieces to this argument, so I will take it one step at a time.
The first argument to deal with is 1)We are short-sighted and incapable of discerning good and evil by ourselves, especially on the Godly level. 2)God is infinitely loving and perfect. 3)Therefore, following his will is the most ethical course of action.
This appears to be a valid argument, but it is internally contradictory. I will set up a dilemma to further elucidate this. Either we are incapable of discerning right and wrong or good and evil by ourselves, or we are capable of this discernment. If we are incapable of determining morality on a Godly level, as the argument suggests, then we are incapable of being able to tell whether or not God is good to begin with. If this is the case, then there is no reason to assume that following his will is an ethical course to take. If we do give ourselves the freedom to say "God is good," however, then we should also have the ability to say "God is bad," if indeed that is the case, and to create our own ethical code. Why should we be able to ascribe goodness to God but not to anything else?
The next argument is 1)Ethics are only subjective or nonexistent if there is no God, but objective if there is. 2)There is a God. 3)God's Ethics are objective and should be followed.
Again, for the sake of argument, I will be pretending that God certainly exists, because these arguments would hold no water otherwise. For something to be objective, it must be based on fact alone or at least be completely external to the mind. It cannot be swayed by prejudice or emotions, nor can it be created out of them. Ethics are never objective. Objectivity deals with the way things are, not with how they ought to be. Normative laws, those subjective laws that dictate how one should behave, are created to the end of obedience. Obedience implies a level of subservience, which in turn implies authority. There is never a reason to follow God's laws if they are not in our best interest. This is because he has no authority over anyone. Authority must be granted by the subordinate party, which means that the subordinate party is actually in control. God only has authority over those who grant it to him; the rest of us are free to do as we please.