The scriptures never suggest that one need not obey the gospel because there will be another chance to do so in the next life. That would suggest that there was very little reason to have the gospel in this life. What the scriptures teach is that there is certainly value in living according to the gospel.
Therefore, Amulek teaches that it is important to act upon the knowledge of the gospel that we receive in this life. His audience has asked for the opportunity to have faith, and they have been taught about the Redeemer in whom they should have faith. Thus, they are placed in a position where they must act. They cannot procrastinate by suggesting that maybe it will be more convenient later.
The important teaching is that “that same spirit which doth possess your bodies at the time that ye go out of this life, that same spirit will have power to possess your body in that eternal world.” We are who we have learned to become in this life. Perhaps there will be some who will be surprised to find themselves in the next life, but if they will be surprised at where they are, they will not be surprised at who they are.
If we are not the kind of person who can dwell in a kingdom of glory, then we will have become the kind of person that cannot dwell in holy temples, and, therefore, we will belong to the spirit of the devil.