When C.S. Lewis referenced abortion he did so off the cuff in a letter talking about how to accommodate for the consequences of sin.
“It is certainly not wrong to try to remove the natural consequences of sin provided the means by which you remove them are not in themselves another sin. (E.g. it is merciful and Christian to remove the natural consequences of fornication by giving the girl a bed in a maternity ward and providing for the child’s keep and education, but wrong to remove them by abortion or infanticide.” – Collected Letters of C. S. Lewis, Vol. III, 91
For a heavy-weight theologian and prolific writer it is telling that this is pretty much the only scrap of writing I’ve ever found of him discussing the topic. I am forced to believe it is because people living in the 40s didn’t debate whether it was wrong or not to have an abortion. Everyone aside from the eugenists of the day pretty much agreed abortion was immoral.
Why? Because they lived in a post-Christian society. Culturally they were all raised to discern good from evil even if they didn’t adhere to the main religion of their day – their parents taught them morality. One can’t press that point enough – when people call certain institutions of our modern era “progressive” they are actually referencing a time before Christianity. They are harking back to a time when paganism ruled the Earth and hearts of men – for abortion was a widely accepted practice in the ancient world. Infanticide too.