What does Romans 6:8 mean?

The word "if" near the start of this verse can be read as "since," as a reference to those who are in Christ. As Paul showed in the previous verse, our faith in Christ began with a spirit-baptism into Christ. This caused us to be so closely identified with Him that God gives us credit for His righteousness and takes His death as payment for our sin.

In that sense, we died with Christ on a spiritual level and were resurrected to a new spiritual life. We are spiritually alive for the first time ever (Ephesians 2:5). Paul now writes that because we died with Christ in that way, we will also live with Him. It's unclear if Paul is referring to living in Christ now as spiritually-resurrected people or living with Christ for eternity as physically-resurrected people. However, both are true, and both might be indicated by this verse. God intends for us to live this new spiritual life He has given to us with Christ. And we definitely look forward to the day we will be physically brought back to life to spend eternity with Christ.