What was the first Christian country in the western world?

Explain how you come to this decision.

That would be very debatable as there is no strict definition for "western world"... but I'd think Armenia would be a good candidate based on what I read here

... or maybe what was called the Roman Empire when Constantine ruled it...

Sorry , but you way off beam.
Try and follow St Paul's journeys and you come up with the right answer.

Rolcam.