Every generation has had critics of Christianity, and each generation has had great men who defended the historical Christian faith. Many of these men presented great arguments, but they have been left unread in remote libraries.
Now a few people have done the hard work of locating these historical apologists and posting them online for free. Two sources are:
Both of these sites have links to a large library of apologetics books from the last 300 years, all public domain and posted online for free. The men who wrote these works present very valuable refutations of critics and skeptics.
Very few of the modern arguments presented by atheists are new. The criticisms have been thrown at the church for hundreds of years, and rightfully answered. Too bad the church has played the ostrich, and put its head in the sand, hiding from the world. Instead, we need Christians who will do their homework and take the truth out to answer the critics questions. Why? So that evangelism can happen more effectively, and because we are commanded to give every man an answer, a reason for the hope that is within us.