How would you respond to someone who claims that Catholicism is nothing more than a man-made religion?

First of all, I think, a man made religion would have crumbled are caused chaos.  It would have misled people grossly.  If you read about the history of the Catholic Church, lot of good things that were given to humanity and society were from the Catholic Church.  This includes the Bible. 

I would ask this person which of the religions that every society since the beginning of recorded history has believed and has sought relationship with the transcendent creator is not man made?

The point of that follow up question is in part to inform the skeptic that the base premise of a transcendent deity is somehow inherent in humanity. If it is natural (part of our nature) to reach out in communication to this non-material creator; this transcendent deity, there must be a non-material dimension to humanity, namely the spiritual. If this spiritual dimension is indeed there, there must be a spiritual source that unites all spiritual nature. If this is true then there must be a right form and practice (orthodoxy) to this relationship. That right form and practice would necessarily then be found in the perfection of truth, goodness and justice because the practice of relationship is a practice in justice; in giving to another what is due, a practice of goodness because it would desire the reach for perfection in the relators as well as the fruits of the relationship and also in truth because if there is a reality to this relationship it must necessarily be true. Being that this would happen between humankind and the creator of humankind, what is due the creator is the highest form of truth, goodness and justice, all three of which do not exist in the empirical order, thereby making them not man-made, but Spirit breathed. Now the question becomes, which one most perfectly fits? Dare we examine?