In the Greek to believe in is to believe that

"Then too some preachers who have had a year or two of Greek make allegedly scholarly remarks about a difference between a New Testament belief and a pagan Greek belief. Better scholars, Gerhard Kittel, Gerhard Friedrich, and Rudolph Bultmann, have these things to say:

From a purely formal standpoint there is nothing very distinctive in the usage of the NT and early Chr. writings as compared with Gr. usage . . . . Pisteu eis is equivalent to Pisteuein oti . . . 'to regard as credible, as true.' Pisteuein eis Christon Ihsoun (Col. 2:16), eis auton and eis eme (often in Jn.) etc, simply means Pisteu ein oti Ihsous apethanen kai aneoth . . . . In Jn. esp. Pisteuein eis and Pisteuein are constantly used interchangeably in the same sense. [Note: Cf. also Ac. 8:37E. . . .] This is proved also by the poss. expression Episteuthh (sc. Ihsous Christos, 1 Tim. 3:16) and the fact that Pistis is equivalent, not to Pistis c. dat., but to Pistis c. gen. obj . . . .

Two pages later they write,

Pisteuw often means to believe God's words. Belief is thus put in Scripture (Jn. 2:22), in what is written in the Law and the prophets (Ac. 24:14), in what the prophets have said (Lk. 24:25) . . . in Moses and his writings (Jn. 5:46f), also in what God is saying at the moment, e.g., through an angel (Lk. 1:20, 45; Ac. 27:25). [See also pages 208 and 222.]

To translate or to summarize a little, these three language scholars said: To believe in is equivalent to believe that. To believe in Christ Jesus simply means to believe that Jesus died and rose again. In John especially to believe in and to believe that are constantly used interchangeably." - Gordon H. Clark, Faith and Saving Faith