What does this sentence mean? (Christian)?

A self-contained or closed system of revelation would mean those theologians draw on the Christian scriptures alone for understanding. They may be Biblical New Testament literalists or simply do not want to go "outside the margins of the text" when understanding Jesus' historical life. For example, the gospels tell us little about Jesus' early childhood. I believe the "from below" language means given Christ's earthly life. The scriptures themselves draw consideration as to Jesus' life after resurrection up into itself since we do not know with certainty, except by faith, who the true historical Jesus was.