Since the events predate the record of them in writing it follows with infallible logic that people did the things written about before the writing took place. So, we know that Jesus lived and taught before the gospel were recorded. We also know that the events recorded in The Acts of the Apostles took place before the book was written, consequently we know that many churches were established before the book of Acts was written. And because churches existed, we know that the handed down their practises and spoke about their beliefs before these were documented in letters, books, or any other writings. Thus, your claim that, "The only way to know that celebrating the Eucharist, baptizing etc was taking place, is because that's what scripture says", is in error because we know of the activities by means of handing them down by example before any scripture describing them or explaining them or even naming them was written.