Steve Ray has a list of 35 loaded Questions for "Bible Christians" (quotation marks his)(link to the whole list). This is number 31/35. I'm trying to provide the answers in a common format, for easy reference.
31) The time interval between the Resurrection and the establishment of the New Testament canon in AD 382 is roughly the same as the interval between the arrival of the Mayflower in America and the present day. Therefore, since the early Christians had no defined New Testament for almost four hundred years, how did they practice sola Scriptura?
One doesn't need a "defined" canon to read the Bible.
1) Obviously, one needs some idea of which books are the Bible, and the early church did have an understanding of that, from the very days that the Scriptures were being penned. For example, Peter referred to Paul's epistles as "Scripture" long before any council convened to "define" the canon (and even before the canon of the New Testament was closed).
2) The definitions of the canon in the late 300's were not "binding" on all Christians. Nevertheless, the Christians of that time remarkably were mostly in agreement as to which books were inspired and which were not.
3) Early Christians used what they had. Some failed to recognize that one or more of the inspired books was inspired, but they used the books that they knew were inspired. Other Christians erroneously thought that other writings were inspired, and mistakenly relied upon them. Nevertheless, like the Bereans they searched the Scripture and established their doctrines from that source.