Well, Keith Urban and Nicole Kidman were married in a church, so if he were atheist or Islamic, I doubt they would have been married in a church.
Secondly, Nicole Kidman was raised Roman Catholic. She has said in recent interviews that she does believe in God. However, she was married to Tom Cruise, a devout Scientologist, for the better part of 9 years. And, Mr. Cruise is somewhat notorious for making potential wives convert to Scientology first (i.e.- current wife Katie Holmes, who was also raised Roman Catholic. She converted to Scientology). The problem is that Scientology does not define "God", so for Nicole Kidman to say that she does believe in God means that she cannot be a follower of Scientology.
Keith Urban has said many times that God is the only reason he made it through his first addiction to drugs and alcohol.
Also, several of Keith's songs have "God" in their title.
The thing is, Keith has said that he will not talked about his "faith", although he has alluded to the fact that he is a Christian many times.
Besides, coming from Australia, gives a small indication that if he is of any faith, chances are it's probably Chritianity, as statistics show that Australia's predominant religion is Christianity.
Keith also performed with Brooks & Dunn on the 2006 Academy of Country Music Awards and they performed Brooks & Dunn's song "Believe", which is without a doubt a song about the Christian faith.
Also, Keith and his mother have given interviews in which both were wearing cross necklaces. Now, granted that doesn't necessarily mean anything this day and time, but it could also be a symbol of their faith.
In the end, though, as someone who strongly believes that your faith is a deeply personal thing, I honor and respect his decision to keep his faith to himself and not wave the banner of any religion in order to gain more fans.
AND, I would hope that no one here who calls themself a Christian would judge him or condemn him for this decision.
GOD BLESS!