From the book that kind of made him famous, the reason for being worried about liberal Protestantism:

The plain fact is that liberalism, whether it be true or false, is no mere “heresy”–no mere divergence at isolated points from Christian teaching. On the contrary it proceeds from a totally different root, and it constitutes, in essentials, a unitary system of its own. That does not mean that all liberals hold all parts of the system, or that Christians who have been affected by liberal teaching at one point have been affected at all points. There is sometimes a salutary lack of logic which prevents the whole of a man’s faith being destroyed when he has given up a part. But the true way in which to examine a spiritual movement is in its logical relations; logic is the great dynamic, and the logical implications of any way of thinking are sooner or later certain to be worked out. And taken as a whole, even as it actually exists today, naturalistic liberalism is a fairly unitary phenomenon; it is tending more and more to eliminate from itself illogical remnants of Christian belief. It differs from Christianity in its view of God, of man, of the seat of authority and of the way of salvation. And it differs from Christianity not only in theology but in the whole of life. It is indeed sometimes said that there can be communion in feeling where communion in thinking is gone, a communion of the heart as distinguished from a communion of the head. But with respect to the present controversy, such a distinction certainly does not apply. On the contrary, in reading the books and listening to the sermons of recent liberal teachers–so untroubled by the problem of sin, so devoid of all sympathy for guilty humanity, so prone to abuse and ridicule the things dearest to the heart of every Christian man–one can only confess that if liberalism is to return into the Christian communion there must be a change of heart fully as much as a change of mind. God grant that such a change of heart may come! But meanwhile the present situation must not be ignored but faced.

Christianity is being attacked from within by a movement which is anti-Christian to the core. (Christianity and Liberalism, 160)