What does baptism of the Spirit ‘into one body’ mean in 1 Corinthians 12?

This text is from a transcript of a question-and-answer session with David Gooding.

The moment you get saved, you become a member of the body of Christ. And that great vast group of persons who are saved, from the Holy Spirit's coming on the day of Pentecost to the rapture, from every kindred, tribe and tongue is referred to in Scripture as 'the church, which is his body' (Ephesians 1:22–23).

Now, in order to try and explain to us what exactly this great vast company of people are to God, God uses certain metaphors and similes. Sometimes he calls them a 'flock' (see John 10:12). Now, we are not really sheep, are we? We're human beings. It's a metaphor when he calls us sheep. In 1 Peter 2:25 it says, 'for you were straying like sheep', and that is a simile. Then sometimes, as in Ephesians 2:21, God calls this great body of people a 'temple'. But we're not really stones, are we? And then sometimes he calls us a 'house', as he does in Hebrews 3:6. But we're not really bricks. In 1 Peter 2:5 we read, 'you yourselves like living stones are being built up as a spiritual house.' So, these are all figures of speech: metaphors and similes intended to help us grasp the truth.

But there is one thing that is most important to learn about this vast company: you never were 'baptized into the flock'. You were never 'baptized into the temple'. You were never 'baptized into the house'. But you were baptized into the body of Christ (see 1 Corinthians 12:13). Now, that is vital. Being baptized into the body of Christ is not a metaphor or a simile; it is an actual fact.

Perhaps that might help to clarify the question, but hours of discussion and careful work would be needed to make sure that we have understood exactly what this means.