Paul is speaking to the Corinthians as if they were one person, so all instances of "you" and the command "be" here are singular. (See: [[rc://en/ta/man/jit/figs-you]])
This can be stated as a statement. Alternate translation: "To anyone who was a slave when God called you to believe, I say this: do not be concerned" (See: [[rc://en/ta/man/jit/figs-rquestion]])