Why does God consider believers special and already seated with Him in heaven?
Because believers believe the gospel, God identifies them with Christ in His death, burial, resurrection, and seating. By grace, not merit, He quickened them together with Christ, raised them up together, and made them sit together in the heavenlies in Christ Jesus, considering them already seated with Him.
Les explains that when Christ was on the cross, in the tomb, and rising in resurrection power, God saw believers there with Him. Paul brings out that believers have been raised, resurrected out of that deadness, and made to sit together in the heavenlies. Believers are so special in God's sight simply because they believe the gospel that He considers them as already seated with Him in the heavenlies. Hence they wait for Him to come and take them to Himself. This also comforts believers concerning loved ones who have died in faith: if they were believers, they are in His presence, already seated in the heavenlies. It is not that big a jump from here to there, so believers do not have to fear death, because men can kill the body but cannot kill that which is already a citizen of heaven. Even when believers were dead in sins, He quickened them together with Christ. It is by grace, not of merit; by grace they are saved. He has raised them up together and made them sit together in the heavenlies in Christ Jesus. Grace is unmerited favor.