The new Covenant that jesus brought about is a simple way we can harmonise with the complicated laws of god and nature. It is not different, it is just simplex. For those who are lacking in truth and understanding, this is a simple foundation apon which we can begin our alignment.
Why aren't Christians under the 10 holy commandments? This is something that Christians should really be asking themselves. When we snap out of a passive state of naivety we start to question things like this which is very healthy. For me, I relate and identify with all of these 10 commandments. But I relate to them through God personally rather than the 10 commandments themselves. This i believe separates what is man and what is truth. Or what is man and what is God.
Why where they given and for how long? Refering to my last answer, Natural law or the laws of God are eternal. Always have and always will be. Never changing, they don't discriminate. They where before the 10 commandments and they will be still here when the 10 commandments are nowhere to be seen. Mans perspective of the 10 commandments, why, who, what, how and for how long becomes irrelevant to the truth or irrelevant to God. God is the source, if it is true, it is true. Truth does not change, only our perspective of the truth.
Are we sinners? So there are two aspects of our reality. The inside world which are emotions, and the outside world which is our physicle reality. Our physicle reality and man itself (Adam) is in a litteral sence, sin manifest. Our bodies are manifestations of sin itself. We can come to this reasoning because something that we can see and comprehend is not perfect. You can't see God. You can't see perfection. Perfection is whole, and the source lf its own life. This we are not and are slowly moving towards death. We are living manifestations of sin. However on the inside world, we are able to transcend our physicle reality and experience the spirit of God, or what is the source of all life and truth. Snapping out of ignorance and naivety and challenging our belief systems, becoming un-comfotable and challenging the aspects of our flesh on the inner world, which are things like belief, ego, pride, controlling nature, possession etc. We align ourself with God on from the inside slowly start to become more and more sinless.