We all have the same God, we just serve him differently. Rivers, lakes, ponds, streams, oceans all have different names, but they all contain water. So do religions have different names, and they all contain truth, expressed in different ways forms and times. It doesn't matter whether you're a Muslim, a Christian, or a Jew. When you believe in God, you should believe that all people are part of one family. If you love God, you can't love only some of his children.
From the scientific view, the theory of karma may be a metaphysical assumption -- but it is no more so than the assumption that all of life is material and originated out of pure chance