Communion (aka The Lord's Supper) is designed to help us remember Jesus' death on the cross. Jesus himself started it with his disciples (Matthew 26:26-29). There is nothing magical about the bread or the juice. They simply serve as reminders for us. The bread reminds us that Jesus' body was brutally beaten and nailed to a cross.
The juice reminds us of the blood that spilled out of Jesus' body when he died for us. We offer the Lord's Supper last sunday of the month because that is what the New Testament church did (Acts 2:42). We do not bar anyone from taking communion. It is the Lord's Supper.
Since it's God's and not ours, we don't think we have any right to forbid anyone from taking part. If you believe in Jesus and wish to take part in communion with us, you're welcome to do so. If you're more comfortable not taking part initially, just pass on the bread and juice trays when they come to you.