Here’s a question for you. At what point did the Son of God —the second person of the Trinity— become a human? That is, the Son of God has always existed with the Father but has not always been a human. So at what point did He become also Human? The answer that leaves you not being a heretic is “when the Holy Spirit came upon Mary and she conceived” (Matt. 1:18). Which gives you all the grounds you need as a Christian to know a child is a human living being from the moment of conception and so abortion is the ending of a human life and a terrible sin.
Think about it like this. Does Jesus become human at conception? If you say no, then The Son of God has to come down and occupy the existing human Jesus body at some point during pregnancy (or afterwards). That’s heresy. It’s saying the Son of God is not truly human but only puts on humanity as a temporary covering. That kind of Jesus could never truly suffer on the cross, because who he is is separate from who is dying and suffering. Also if he was just temporarily putting on a human body, the resurrection would be nonsensical. Why does he need a new body if his old one was always some temporary structure he was only using? That makes the resurrection less God raising Christ from the dead and more God confining him back to a body again.
But, you could argue, that every person becomes a human —gets a soul— at a certain point of development, let’s say 24 weeks in. And so there’s nothing wrong therefore with the Son of God becoming this child at 24 weeks in. I think this still falls down for the same reason as above, then Christ is merely slipping into human flesh and doesn’t go through everything we go through, but let’s run with it for now. If this is the case, what is the child before 24 weeks? It can’t be the Son of God because He hasn’t become this child yet. But it can’t just be a regular human embryo because it’s conceived by the Holy Spirit. And so what then is it? A little bit God? Could Mary then have had an abortion up to 24 weeks, because while that embryo was going to be God, it wasn’t God yet, so it would have been okay to get rid of it? That doesn’t seem particularly possible, and that’s before you get to thinking about whether any sensations the embryo would have before Jesus came and occupied it would be part of Jesus’ existence or not.
So, let’s say you say yes then, yes Jesus becomes a human at conception. Then you’re saying that conception is the time that a person starts to exist (or in Jesus’ case, a person starts to exist as a human). If Jesus becomes a human life at conception then that’s the time that human life must begin for everyone. Unless of course Jesus is an exception to the rule. But if he’s an exception to normal humanity then again you’re guilty of heresy. Because what you’re saying is Jesus didn’t become a human like everyone else, didn’t go through what everyone else did, didn’t have the same weaknesses as he did. But the Bible is clear, he became a human like the rest of us so he could save us. And so the Jesus child in the womb must go through exactly the same process
Now, I confess, it boggles the mind slightly to think of the eternal Son of the Father, the Word by which the world was spoken into being, being a tiny zygote and yet still remaining the Son of God. Partly it’s because we’re so worldly people. We struggle to think of anything beyond the physical. The idea of a soul that survives death doesn’t sit easy with us, neither does the idea of a soul that exists before the mind has formed, but the Bible says that a soul exists outside the body (Matt. 10:28) and we should trust it. But more than that, how can we fully comprehend a God who would become so low to save us? That God would send his son in such weak and pitiful flesh, so at risk to disease and illness? And yet he did.