I have an agnostic friend who was raised Catholic and believes that 'something' is 'up there' and created all of this. But he is convinced that all religion is man-made, whose purpose is behavior control and extortion. I sense that he is searching for something to hope in. Where do I begin to demonstrate the proof of God that is in the pages of Scripture? How do I demonstrate the truth of unperverted Christianity compared with man-made denominationalist religion? I don't want to botch it up and would appreciate guidance.
There are too many ways to approach this situation because we don't know why your friend is choosing to espouse a belief that is contrary to the evidence around him. "For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse, because, although they knew God, they did not glorify Him as God, nor were thankful, but became futile in their thoughts, and their foolish hearts were darkened" (Romans 1:20-21). His agnostic beliefs are the result of the choices he has made. You set the result, but correct the disbelief you need some idea as to why.
I know a number of disgruntled Catholics. Some have the idea that if you have seen one religion, you have seen them all. It could be that in rejecting Catholicism, he has over-reacted and rejected all religion. More often, though, you find that "religion" has become the bad guy in a person's mind. It restricts something a person feels he has the right to do; therefore, it is easier to remove religion than to change personal behavior. I met one young man, years ago, who was living with a girl. When I approached the subject of why his situation was wrong, he would throughout some excuse as to why he didn't think the Bible was true. I would address the issue and he would agree that his argument wasn't that great. But as soon as we got back to the original discussion of why fornication was wrong, out would come another excuse as to why the Bible couldn't be trusted. I soon realized that it wasn't that he thought the Bible couldn't be trusted, he didn't want to trust the Bible because to do so would mean having to change his behavior.
The approach you need to take all depends on the reason he has pushed God of his life. Generally, a discussion about why things are right or wrong, or a discussion about why God would make a world only to ignore it (i.e. what is the purpose of life), will often let you know where the difficulty lies.