Actually that has not been proven by any means. Early church fathers all had a post tribulation position and THAT has been proven multiple times and is still evident today (not like any of them can change their minds now anyway) :).
The history of the pre-trib doctrine can clearly be traced back to a Morgan Edwards book in 1788, then in the early 1800s to Edward Irving from 1825-1832 where he first began teaching this ‘secret’ rapture. Darby only picked it up shortly after that and was refuted in writings from notables such as Samuel P. Tregelles (1813/1875) in 1855 and William Kelly (1811/1888) also around that time who both clearly identified the ‘Irvingites’ as those that first brought forth this doctrine.
Even after a 20 year search of early church writers, Grant Jeffrey failed to produce one actual true example of support for a secret rapture. Shamefully he did take about a half dozen quotes out of context as proof but these were quickly and embarrassingly disproved.
JB
The first person who formulated this eschatology was a Jesuit named Ribera in 1591AD. He interpreted the book of Revelation suggesting an end-time personal antichrist, a rebuilt Babylon and a rebuilt temple in Jerusalem at the end of the Christian Era.
shalom b'SHEM Yah'shua HaMashiach Adonai