That one is simple. Romans is written to "those called to be saints"; the church. It deals with the necessary belief in the resurrection of Jesus for a child of God to be saved.
It is addressed in other epistles as well. As the church grew and people came into the church, some carried the baggage of past beliefs, such as the belief in no resurrection.
It is also dealt with in
1 Thess. 4:13-18.