The Church has not declared whether Mary died a physical death or not. I think most sources or opinions is that she did die, but was subsequently assumed into heaven, body and soul. (since Christ died - more coming up on this)
The explanation for Mary's Assumption is based on the concept of Consortium. This is based on Scripture:
"And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with [him], that we may be also glorified together". Romans 8:17
Since Mary participated in the Redemptive work (not that she was necessary, but God willed her being there and taking part), including the Suffering at Calvary, (SHE was ALSO pierced with a "sword" at the foot of the cross, as predicted by Simeon) she would also be glorified. Being the first hearer and believer of the Word, naturally, she would be the prototype of what would become of believers who followed after her.
Also if she were without original sin; than she would neither want nor need Christ's redemption in any form.
Mary needed salvation from sin just like any other human. However, God acted on her in a singular way, freeing her from original sin at her conception. Mary herself proclaims that God is her savior in her Magnificat:
"And my spirit hath rejoiced in God my Saviour". Luke 1:47
God saved her by preventing her from falling into sin.
Regards
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?