The encroachment of liberal theology into conservative churches of all kinds has been a long, complex process, in which there were many players over a very long period of time, not the least of whom was one Thomas Aquinas and his unfortunate elevation of reason in relation to faith. No, I am not blaming him. No one person or group provides that single point of inflection on which the whole matter turned. To get an idea of the full scope of the problem, I highly recommend a book by Francis Schaeffer called Escape from Reason. Schaeffer was a Presbyterian, but his explanation of the history of western thought in relation to Christian theology is a powerful antidote to the temptation to make incendiary oversimplifications.
Thank you kindly,
~Theo