Thank you for your explanation.
But wasn't Mary still alive when Stephen was martered?
Would that not make him the first Christian in heaven?
I think tradition states that she lived with John for quite some time, so I would agree that Mary was still alive.
As to Stephen being the first Christian, perhaps. First, some people may have died naturally, after converting to Christ, before St. Stephen was stoned. I am not sure where the "Good thief" stands on this, either. I would say St. Stephen was the first who willingly gave his life for the Christian Gospel.
Regards
I think the thief on the cross next to Jesus (the one who asked 'Remember me when you come into your kingdom'), to whom Jesus promised 'Today thou shalt be with me in Paradise', would qualify as the first Christian into Paradise with Christ. But to Heaven, well, there were forty days Jesus stayed around the Earth after His resurrection and prior to His ascension. One would be hard pressed to offer proof of whom may or may not have died 'in Christ' during that period prior to and at His ascension.
Our Lady is the first to be in heaven in both body and soul. Because Our Lady was prevented from ever receiving the stain of original sin by a unique grace and committed no personal sin her body would not suffer corruption. The rest of the souls in heaven will not be united to their bodies (which will be glorified) until final judgment on the last day.