While it might be Christian justices that made the ruling, its not a question of when life begins per God (there is no Biblical scripture directly saying when life begins) , it's when life begins per biology.
An embryo at 2 weeks already has every cell (a cell, which is the basic unit of LIFE by the way) that would form the relevant organs in the human body, it has DNA distinct from both parents (or the man and the woman) at 4 weeks it already has a beating heart and has started to form limbs at 5 weeks and at 8 weeks it has assumed a basic human shape.
So why again shouldn't this life be protected? So what "my body my choice" is saying is that when the woman feels like, what she carries is a baby to be protected at all costs and when she feels like, it is something she can discard, it is just a useless clump of cells. Does this not sound strange? So a woman is entitled to choose who lives and who doesn't based on how she feels simply because it's growing in her body?
And for the most part girls don't get pregnant, women do. For the girl Biden made reference to (if she exists at all because I honestly don't know) that's a sad thing if it is true, however that is a detour from the main point of the prochoice message, given statistically 1% of abortions are due to rape and about 0.5% due to incest, these are side issues from the main pro choice clamour. The real issue is they want to have sex as they please but don't want to deal with babies when they come as a result of said sex.
So in conclusion, no, that is not a religious ruling as you say, unless you think the state protecting human life is religious as well.