Dear Bob,
We are Christian upon believing that "Christ is the Son of God".
That is the Ancient and first creed of Christian belief.
Upon stating that, a person (or an entire family) was baptized.
A baptized believer was then trained in Christianity. Normally the parents if it were a family.
And finally allowed to partake in what is known as the liturgy of the believers, which is a closed door service of which there are many.
In the liturgy of the believers is the "Anaphora" prayers, where Christians ask that The Holy Spirit descend and change the bread and wine offered into the body and blood of Christ, which is then distributed at the hands of those leading the "Anaphora" prayers.
Anaphora - OrthodoxWiki
While some might say it's being exclusionary, we think of it as "Christian's in training". The individual who is untrained is what we today call a catechumen, they are prayed for fully by the Church, even so much as to be buried with full Christian rites if one should die before completion. (This is in the day before bibles and training a Christian could take a while.)
One who is not fully trained is still a Christian upon their profession of faith in Christ as the son of God, but not fully "in" The Church.
I hope that helps. No need to feel excluded.
God be gracious to me a sinner.