Does the 'we all' in Paul's letters mean everyone will be saved?

No. In Paul's letters, "we all" refers to believers, not all humanity. Paul never writes to anyone but the believer, so "we all" includes every believer and excludes lost people; it does not teach universal salvation. The phrase concerns all believers being baptized by one Spirit into Christ's body.

Les explains that a woman once asked whether this meant everybody was going to be saved, but she had not been taught that Paul never writes to anyone but the believer. When Paul says "you all," he is not including lost people; he is including every believer, and that is as far as it can go. The verse says, "For by one Spirit are we all baptized into"—not a local church, not a denomination—but into the body. This is an act of God, invisible, not something we can put our hands on. We know it happened by faith because the book says so. At the moment every child of God believed, the Holy Spirit baptized them into the body of Christ.

When Paul says 'you all,' does he include lost people or only believers?

When Paul writes "you all," he includes only believers, not lost people. Paul never writes to anyone but the believer, so the phrase reaches every believer and goes no further. The context is the Holy Spirit baptizing all believers into the body of Christ, not a local church or denomination.