If Bible alone is the way to go, then what did the Christians do for 400 years before there was a Bible. There were tons of writings called scripture. Many more books than there are in the Bible.
Which is what the Bereans were searching to see if Rav Sha'ul was telling the truth. You seem to dismiss the Tanach.
WHY . . .
i GUESS they HAD to trust . . . drum roll . . .
THE HOLY SPIRIT!
What a novel idea!
They had NO MAGICSTERICAL of the looming heavy handed political sort monopolizing on all the power mongering they could . . . increasingly with added doctrines of men; doctrines of demons . . . one brazen political ploy after another . . .
Just them and Holy Spirit and servant hearted, often itinerate leaders . . .
And God was with them.
PTL.