Modern Catholics and evangelical protestants have largely taken the position that personhood accrues to a zygote as soon as fertilization occurs. However, it is clear from their other positions and behaviors that this belief is only for the purpose of making the anti-abortion argument, and does not reflect a true belief in the personhood of a zygote/blastocyst in the relevant sense.
For example, reviewing the policies supported by anti-abortion religious groups, one is struck with how little they accord with the stated position that a zygote/blastocyst is a person, and the destruction of a zygote a murder; what the supported policies all appear consistent with is a desire to punish women who have sex. In the "moral foundations" thinking of Jonathan Haidt, we might say that concerns for authority and for purity are masquerading as concern for fairness or for harm/care.
In addition to punishing women who have sex, anti-abortion religious people also want to force everyone to submit to the will of their God - concern for authority and in-group loyalty again masquerading as concern for fairness and harm/care.
Imagine for a second that a zygote really is a person, whatever that means to you.
Now realize that up to a quarter of all recognized pregnancies, and HALF of all fertilized zygotes (some sources say up to 70%), are spontaneously miscarried.
That's a lot of death. If you take a zygote to be a person, half of all children are dying before they ever take a breath.
But "No pro-life group has called for the foundation of a National Institute for the Prevention of Miscarriage," notes a commenter in the above-linked thread.
Of course, intentional harm is both more deserving of blame than unintentional harm and more preventable than unintentional harm. But the assignment of, apparently, ZERO value to the accidental deaths of billions of zygotes (after all, God willed it) while claiming that the intentional killing of these zygotes is MURDER is hardly consistent.
- Religious people do not believe that zygotes or blastocysts are persons.
- Religious people want to punish women for having sex.
- Religious people want to force everyone to submit to their God.
See also: Five Reasons to Have an Abortion