Because there is one loaf, we, who are many, are one body, for we all share the one loaf. (I Corinthians 10:17)
Taking communion is an act of Christian unity. Our unity is symbolized by the taking from one loaf, which represents that all true Christians partake of the blessings from one man who sacrificed His body for us, Jesus Christ.
Sadly, there are some churches that complicate our understanding of communion by adding their own man-made rules about who should be allowed to take it. These false opinions fall into two general categories.
The LIBERAL VIEW essentially says that everyone and anyone should be freely offered communion. That is wrong. Communion is only for Christians. Church attenders should be clearly warned not to participate if they are not believers.
the CONSERVATIVE VIEW, seeks to counter the errors of the liberal view by excluding everyone who does not belong to their specific branch of Christianity. For instance, a conservative Lutheran church will not offer communion to a Baptist, and some Baptist churches might do the same to someone who has been baptized only as an infant. I believe this is wrong. Communion should be offered to all true Christians. We can exclude someone for living an extremely immoral life or adheres to extremely heretical views, but we must not exclude believers from other denominations than ourselves, because they belong to the same body as us. There is only one Church, and it transcends denominational barriers.