The fact is, we don't even know who wrote the Gospels, but you take on faith that they were inspired by God. On what do you base that faith? Scripture?
The universal testimony of the church.
This is an oxymoron. The mere existence of Protestantism belies any assertion of "universal testimony" outside the Catholic Church - "catholic", of course, meaning " universal.
In fact, one could say that Protestantism is a collection of many thousands of "parallel" universes, with no two testimonies coinciding...
The universal testimony of the church.
In other words, Sacred Tradition.
SD