"Can I be a Christian without joining the church?"
Answer: Yes, it is as possible as being:
A student who will not go to school.
A soldier who will not join an army.
A citizen who does not pay taxes or vote.
A salesman with no customers.
An explorer with no base camp.
A seaman on a ship without a crew.
A business man on a deserted island.
An author without readers.
A tuba player without an orchestra.
A parent without a family.
A football player without a team.
A politician who is a hermit.
A scientist who does not share his findings.
A bee without a hive.