Question:Why do most scientist reject the possibility of God`s existence? Why do they embrace evolution?
Answer:Two really good questions! First, it is simply not true that most scientists reject the possibility of the existence of God. Many do, but more do not. This is a stereotype which is created largely because the atheist scientists tend to speak much louder than believing scientists. Surveys of scientists support my contention that the number who do not accept even the possibility of God's existence is smaller than most people would assume.
Let me share an anecdote, then some actual statistics. I am a college professor in a chemistry department. In my department, with seven full-time professors, four are believers, two are hard to categorize, but are probably agnostic, and one is an atheist. As for reliable data (as opposed to my unscientific survey), a recent poll by Edward J. Larson of the University of Georgia of active scientists found that 40 percent said that they definitely believe in a theistic God and fifteen percent had no particular opinion while 45% said they did not believe in the God as defined by the survey. The interesting thing is that they asked the same questions used in a poll of American scientists in 1916 and the result was almost identical to the 1916 survey. Add to this that the definition of God used in the survey was a theistic one--a God who acts in the universe. If they had worded the question to allow for an impersonal Creator (consistent with deism) or a pantheistic sort of impersonal universe-pervading "god" then the percentage of those who accept the reality of some sort of supernatural would almost certainly be over fifty percent. In addition, the survey at the University of Georgia assessed only active scientists in research settings. I know from experience, that scientist who are teachers are more likely to be believers, which would increase the numbers even more.
Although your question is based on an incorrect premise (that most scientists reject the possibility of God's existence), it still is true that more scientists do not believe than the general population. For example, in a recent Gallup poll, 93% of Americans have some sort of a belief in God. If true, then scientists have a rate of rejecting the existence of God about five times higher than the American population as a whole. My answer is that I do not know for sure, but I can make a few guesses. First of all, in the process of atending our universities, there is a self-selecting process in which atheists may choose to pursue science and some believers self-select to not be scientists. This is particularly true in biology and neuroscience where so many professors who are atheists are very agressive in their atheism. I have many personal experiences of atheist scientists ridiculing belief--especially biologists. Another reason is that the scientific approach to asking and answering questions about cause and effect in the universe rules out, by definition, supernatural intervention. As a believing scientist, I, too, do not propose supernatural explanations in my science work because, by definition, science only studies natural laws which, by definition, does not include the supernatural.
What I can say for sure it is not because the evidence from science tends toward unbelief. As I have taught many times in my public presentations, (see the Science and God audio I just posted a couple of weeks ago at the web site), the evidence from science points very strongly toward a Creator. The existence of life, the apparent fine-tuning of the universe, the vast array of evidence for design, from the properties of water to the properties of the elements, the existence of a genetic code and many, many more, makes the evidence from science overwhelmingly support the conclusion that God is real. So, what I can say for sure about your question is that although less than half of scientists reject any possible idea of God, the higher ratio of unbelievers in science than in the population in general is not because science itself points against belief in God.
On your second question, "Why do scientits embrace evolution", my answer is very different. The reason almost all scientists "embrace" evolution is that the evidence very strongly supports this theory. We need to be careful about our definitions. The word "embrace" is a rather strong word. For myself, I am not sure that I "embrace" evolution. That sounds like I am a believer in evolution, as if it is a world view to get behind. What I would say is that I accept evolution to be a good scientific theory. Let me state this more carefully. The vast majority of scientists, including the vast majority of Christian scientists, accept the obvious fact, which is that the theory of evolution is a very successful theory. It is a successful theory because it is the only one which is consistent with the data with regard to both the fossil record and the DNA evidence. The fossil evidence, generally, is consistent with the theory of evolution. I know of no fossil discovery which can disprove the general premise of common descent. Missing links still exist, of course, but missing links do not disprove evolution. The DNA evidence even more strongly supports the idea of common descent. As a believer and as a scientist, I find that the theory of evolution fits every single qualification to make it a good theory.
Besides, I find no contradiction between the idea that species have evolved and the teaching of the Bible. The Bible says that all species were created by God, but it does not specify the process he used. I believe that science supports the conclusion that life was created. Also, as a believer, I believe that man was a special creation. In addition, I believe the scientific evidence supports the possibility that God has intervened in the process of evolution of species. It seems to me that God created a universe within which evolution could happen. If evolution was God's idea, then I do not see why Christians should be against the theory of evolution. What we are against is the idea that God had nothing to do with the creation of life or the process of evolution. As a Christian, I find no reason to reject the theory of evolution. While I do not exactly "embrace" it as I "embrace" God, I accept this theory. I believe this will answer your question, if you will allow me to change your question to "Why do most scientists accept the theory of evolution?"