In what sense were believers buried with Christ?
Believers were buried with Christ in the sense that God saw every true believer in Christ in the grave. As Christ was buried, they were buried; as He rose in resurrection power, they also are resurrected in newness of life.
Les teaches that God saw every true believer in Christ as He hung on the cross, saw Him in the grave, and saw us in Christ when He rose in resurrection power. So as Christ was buried, we were buried, and we also are resurrected in newness of life. Paul then explains this from the believer's point of view: the old Adamic nature still influences us, and we must wrestle against it, even though God has reckoned it crucified and declared it dead. In experience it is still with us, but it is defeated and placed under subjection to the new nature that comes by the power of God. This is illustrated by the two natures, as also seen in Galatians 5:16-17.