Paul continues to talk of the variety of gifts God gives believers, God gives different gifts to different believers, but Paul wants them to know that all believers are made into one body, which is called the body of Christ. For this reason believers should have unity.
Possible meanings are 1) the Holy Spirit is the one who baptizes us, “For one Spirit baptized us” or 2) that the Spirit, like the water of baptism, is the medium through which we are baptized into the body, “For in one Spirit we were all baptized” (See: rc://en/ta/man/translate/figs-activepassive and rc://en/ta/man/translate/figs-metaphor)
“Bound” here is a metonym for “slaves.” AT: “whether slave-people or free-people” (See: rc://en/ta/man/translate/figs-metonomy)
This can be stated in active form. AT: “God gave all of us the same Spirit, and we share the Spirit as people might share a drink” (See: rc://en/ta/man/translate/figs-activepassive and rc://en/ta/man/translate/figs-metaphor)