To Believe in is to be believe that

"While Professor Berkhof serves as a good example, many other Protestant theologians also, both Lutheran and Reformed, tend to make a sharp distinction between 'a confident resting on a person' and 'the assent given to a testimony.' 'Confident reliance' is supposed to differ from 'intellectual assent.'
The term resting or reliance is seldom if ever explained in theology books. One  is left in the dark as to what it means. An illustration may furnish a clue and make the words intelligible. Suppose a high school student is assigned a problem in geometry. He works out a solution, looks at it from all angles, perhaps he corrects a small detail and then tests each step again to see if he has made a mistake; seeing none, he now puts down his pencil and rests. That is to say, he has assented to his argument. He believes he now has the truth.
But most theologians are not so clear, nor can they, as earlier indicated, bolster up their imagined distinction with references to pisteuein eis, for a few paragraphs back Kittel disposed of such a contention. English also has the same usage.As Modernism developed in the 1920's and suspicion attatched to this or that minister, people would ask, Does he believe in the virgin birth? Does he believe in the atonement? They did not ask, Does he believe the virgin birth? The preposition in was regularly used. But of course the meaning was, Does he believe that the virgin birth is true? Does he believe that Christ's death was a substitutionary sacrifice? Thus, to believe in a person is to be confident, i.e., believe that he will continue to tell the truth." - Gordon H. Clark, Faith and Saving Faith.