A child can be disobedient in one of two ways. First, he can outright rebel. He can insist on his own way, marching to his own drum, being his own boss. The other way is bit harder to notice, but just as dangerous. He can never grow up. A child is taught obedience not so that he might always be a child, nor so that he can become an entirely independent entity. Rather, so that he might become his own man, but a man in fellowship. Willful immaturity and rebellious autonomy are both sinful.
These opening lines of Hebrews were written to believing Jews who were bracing for looming persecution. They are told that whereas God had spoken in times past through prophets (Heb. 1:1), He had now spoken by His Son, who is the appointed heir of the whole kit and caboodle. Not only that, but the Son was eternally with the Father in the act of Creation (Heb. 1:2). The long and short of it is that the God who had spoken creation into being, is the very same God who now spoke by the Son. This demands precise and faithful adherence to the Son’s kingship (Heb. 2:1). The Triune God spoke creation into being, and the Triune God has now spoken a new creation into being. We know this because God became a man, purged our sins, and became King of the world (Heb. 1:3). This was all a warning to them to not return to the incomplete word of Moses, but to hear the fulfillment of what God spoke through Moses and the prophets by His Son. The better Word had been spoken, and with blood red finality.