Babies become children of God at the moment of conception. They are never (scientificaly or theologically) a “pulp” of tissuse since they are unique human beings as soon as conception occurs.
They become Catholic (capital C) when they are Baptized.
The instant of conception. There is not a human life before, and there is unique human DNA ever after. Anyway, let us look at this from God’s view. At any point, Jesus just a lump of tissue in Mary’s womb?
Consider: John the Baptist, from the womb, recognized Jesus in the womb as well as the blessedness of Mary, upon hearing Mary’s greeting (Luke 1:39-45). Mary went to visit Elizabeth immediately after Jesus was concevied in her womb, so He was no more than a few cells. So, rather clearly, conception is it.
Corki is right: they become Christian, and Catholic, at their baptism. Even if they are baptized outside of the Catholic faith, they are still a member of the universal Church - just not in perfect communion.
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