My understanding of the history is that Constantine was not 100% pious. He was first and foremost the Emperor of Rome and Christianity was a useful tool. I am not saying that compilation and writing of the bible by the council of Nicaea was a cynical political stunt, but in codifying and standarising the christian creed Constantine's political ends were also advanced.
It is also important to note that Constantine did not accept baptism until he was near death. That is, Constantine was free to do whatever an Emperor needed to do to maintain order in the knowledge that all his sins would be washed away before his death.
As for the bible being literally or even figuratively true, I doubt it very much. I don't believe that the bible has anything to tell us about evolution, either for or against. And, while The Origin of the Species is a seminal work on evolution, it is not the best book to read on the subject. The science of genetics had not been invented at this stage, and I don't think it is possible to properly understand evolution in the absence of a consideration of genetics. I recommend The Selfish Gene by Richard Dawkins which deals with both subjects, and also explains how self replicating molecules can spontaneously come into existence in the primordial oceans.
As for scientists believing in god, science can neither prove nor disprove god's existence. It is easy to pick holes in the bible or any other religious text, but that doesn't disprove the existence of a creator.
The basis of science is to test a hypothesis for the purpose of disproving it. If the hypothesis cannot be disproved and it serves to explain and predict observable data, it is accepted as valid until a better hypothesis comes along. While certain generally accepted scientific theories may be incompatible with the doctrines of many or all of the established religions, that only proves that the doctrines themselves are incorrect. It does not speak to the question of a divine and unknowable creator.
