Flatty10, I’m not sure if you’re questioning Mary being Immaculately conceived or if you’re questioning her perpetual virginity as well?
In the Bible Mary is addressed by the angel Gabriel as being the “favored one” (Luke 1.28). Likewise she is referred to as “blessed among women” by Elizabeth (Luke 1.42). During the annunciation Mary herself is shocked by Gabriel’s message that she will bear a son because she is a virgin (Luke 1.34). Her greeting to Elizabeth sanctifies John the Baptist in his mother’s womb as well (Luke 1.44), which is a further indication of her blessed state. In Matthew’s gospel Joseph is also told that Mary, though pregnant, is a virgin (Matthew 1.20).
It would have been much easier for the early Christians to simply have said Mary had normal relations with her husband Joseph and conceived a child who God then sanctified than it was for them to insist on the virgin birth. For example, John the Baptist’s birth was a holy birth, influenced by God, yet he was still born the way all of us are. Yet in his lifetime some thought he was the Messiah despite this fact. So it’s not as if the early Christians couldn’t have convinced people that Jesus was the Messiah unless it involved a virgin birth. But from the start Christianity insisted on Mary’s virginity and her Blessedness. If this wasn’t something these folks truly believed in and felt to be essential to their faith then I doubt they’d have stuck to such an incredible claim as the virgin birth.
Likewise there are gnostic gospels from the early life of Christiantiy that further attest to Mary’s blessed state, from her being born without sin to her being a virgin all her life. The one I am thinking of in particular is the Protoevangelium of James (also called the Infancy Gospel of James), which dates to around 150 AD. Now I know the gnostic gospels are not cannon, but I mention them to illustrate that this belief was common among Christians from the very beginning.
And to make two further points:
Mankind was orignally made without sin. Adam and Eve were both without sin until they gave in to temptation. Now whether you take the Genesis story literally or figuratively I believe one message in that story is clear: mankind originally came into being through God without sin. So if you can believe that mankind began without sin, then how hard is it to believe that God could yet again create a being (Mary) without sin?
I also agree with Apryl regarding Mary needing to be a “pure vessel”, so to speak, in order to carry the Incarnation in her womb.
In the end I suppose none of this will convince you if you are not open to the possibility of it all being true. However, if you are Christian then you must believe in many things that are hard for many to believe in. The Resurrection for starters. Not to mention things like: Jesus walking on water, Jesus turning water into wine, Jesus healing people with a touch, Jesus raising people from the dead, etc. And even his disciples (who were certainly not born without sin) were able to perfom many miracles which would be hard for the skeptic to believe in.