“…who died for us, that whether we wake or sleep, we should live together with Him“
The foundation of our assurance before God is the person and work of Christ.
who died for us,
The phrase “who died for us” modifies “through our Lord Jesus Christ” of the previous verse. Paul points to the sweeping scope of salvation we have in Christ.
The word “for” in the phrase “for us” means on our behalf. Jesus died on our behalf on the cross. He suffered all that needs to be suffered for our sins.
that whether we wake or sleep,
The idea of “wake or sleep” in this context refers to spiritual vigilance versus spiritual softness (1 Thessalonians 5:6,7) rather than to life and death.
we should live together with Him
It makes no difference whether we are spiritual or carnal; the Lord will Rapture us into His presence. This argues against the partial rapture theory that claims that God will translate only the spiritual at the Rapture. The Rapture will shock some carnal Christians living in sin, as God translates them to Heaven right in the act of their sin.
We owe our salvation exclusively to the death of Christ on the cross.
God will not judge us because He has already judged Christ for our sins. We owe our salvation exclusively to the work of Christ on the cross. Christians will never face the judgment of God.
“Most assuredly, I say to you, he who hears My word and believes in Him who sent Me has everlasting life, and shall not come into judgment, but has passed from death into life. “Most assuredly, I say to you, the hour is coming, and now is, when the dead will hear the voice of the Son of God; and those who hear will live” (John 5:24-25).
“There is therefore now no condemnation to those who are in Christ Jesus, who do not walk according to the flesh, but according to the Spirit” (Romans 8:1).