A hard concept sometimes to understand, for non-Christians and even Christians alike, is how the Bible is separated into the old and new testament.
We live in a new testament world, since Jesus's birth. This doesn't wipe away everything the old testament said, but it does change some of the rules and what is and is not sin.
For example, when Jesus was on Earth, he proclaimed that now Jewish people can eat the meats that the old testament said were unclean. Jesus proclaimed those meats now clean, so Jewish people can now eat them. Of course Jewish people don't believe Jesus is the messiah, so they don't believe that, they still operate on the rule from the old testament.
My point is, when Jesus was born, and since Christians believe he is the messiah, a lot of the rules changed from the old testament so that now we live in the new testament world. So the new testament is what we live under now, while the old testament is there as more of a guide to how things were before Jesus.
A lot of things done in the old testament had a purpose, and when that purpose was served the rule was changed when Jesus came to Earth.
Jesus changed everything, it was a big deal for how we live today. Hope that helps explain the differences
