You yourself are evidence of God. You are a wonderful, beautiful creaure with dignity and purpose made in the image of God. You are not a higher animal. You are more than physiological impulses.
Regarding utilitarianism, do you really wish to construct a system of reality based on pleasure and pain? Consider the consequences of a world lived along those lines. Perhaps I consider it pleasurable to have an affair with a girl at the office because my wife will never know. Perhaps I consider it pleasurable to beat someone half to death.
You may say, "Well, that causes someone else pain, so I wouldn't do it." But what if I decide that I really don't care what pain it causes others? What if I decide that the pain it may cause me later is nothing compared to the pleasure it gives me now? What if I don't? Can you speak a word against me? On what basis? With what evidence?
No, since you seem a decent fellow, and I am confident that you are not writing me from the confines of a mental institution, I must say that you do not live out your worldview.
Consider that you do not because somewhere within you there is a yearning for the eternal.
I think that this reality already exist. The activities of our lives are planned around the possible, or known, effects of pleasure or pain from a given cause.
Perhaps I consider it pleasurable to have an affair with a girl at the office because my wife will never know. Perhaps I consider it pleasurable to beat someone half to death.
You may find them pleasurable, but does that make them right? Pleasure and pain are not purely subjective materials, but they are things that, to some reasonable extent, can be measured. If you cause more pain than pleasure, as in the instance of adultery or beatings, then you have committed a wrong, regardless of whether you, personally, find it pleasurable, because the sum total of difference between pleasure and pain weighs more heavily on the side of pain.
But what if I decide that I really don't care what pain it causes others? What if I decide that the pain it may cause me later is nothing compared to the pleasure it gives me now? What if I don't? Can you speak a word against me? On what basis? With what evidence?
If I commit those same acts and do not show care for others, can you speak a word against me, on what basis and with what evidence? Will you use the Bible? What if I do not believe the Bible?
Utilitarianism does not assume that people will always choose to bring pleasure to others; it is realistic in that it assumes people will miscalculate, as in you scenario, believing that some temporary pleasure is greater than lasting pain. We all do this, everyday; we act on some temporary pleasure, sometimes knowing full well that the end result will be greater pain or discomfort. One of the aims of utilitarianism is the inculcation, through some type of moral education, of a caring for others. It is not necessary that it be utilitarianism specifically that is taught. This same moral education of caring for others can be taught through the Christian religion and various other religions.
Can I speak a word against you; on what basis? Most certainly, as you have done harm to another human being, by maximizing their amount of pain. It is premised on the basis of the principle of utility - "...that principle which approves or disapproves of every action whatsoever, according to the tendency which it appears to have to augment or diminish the happiness of the party whose interest is on question." With what evidence? The emprirical evidence which is before us; that pain which has been caused by you and the pleasure that you may have received.
If you have not already read about this theory, you can go to my profile site, where I have links to Bentham's Principles of Morals and Legislation and Mill's Utilitarianism.