How is it ambiguous? The church is the body of Christ. Once you accept Christ as Lord and Savior, you become a member. Pretty straight forward, obvious, and well documented in Scripture.
sigh
Also straight forward, obvious, and well documented in Scripture (as well as in the entire concept of the epistle) is that Jesus established one Church, gave its leaders power to forgive sins, passed their authority on to new leaders as situations warranted, and remained united in one faith, one bread, one cup.
And that Jesus prayed for the Church to remain one.
SD