This phrase does not mean Paul doubts that some of them have God's Spirit. Paul wants them to realize that they all have God's Spirit. AT: "since" or "because"
How Christ lives in a person could be made explicit. AT: "If Christ lives in you through the Holy Spirit" (See: [[rc://en/ta/man/translate/figs-explicit]])
Possible meanings are 1) a person is spiritually dead to the power of sin or 2) the physical body will still die because of sin. (See: [[rc://en/ta/man/translate/figs-idiom]])
Possible meanings are 1) a person is spiritually alive because God has given him power to do what is right or 2) God will bring the person back to life after he dies because God is righteous and gives believers eternal life. (See: [[rc://en/ta/man/translate/figs-idiom]])