That’s an excellent point. If they didn’t have these Church Councils guided by the Holy Spirit, they probably would have just shoved all those other extra-Biblical documents that were floating around back then into the New Testament too. (Like you are saying, the “New Agers” of today are inadvertently highlighting the importance and necessity of those early Church Councils.)
It should also be pointed out that the Bible did not exist in the form of a single volume as it does today, but as separate scrolls and codices each containing one book. And they were fairly expensive, since they were handwritten, even though —astonishingly—looking as though they were printed, so great was the skills of the scribes. Even a Church might not have all the books of the Bible. Until the 4th century, many were not likely to have St.John’s Revelation. In the 4th Century the Emperor did order the production of tens of bound volumes of Scripture. So probably some of the more important churches had already done the same thing. But not until the 380s do we find a definitive listing of the books of the canon.