Deep down, everyone knows that abortion is not a religious issue. One need only look at the surprisingly large number of pro-life atheists and agnostics to know this, but perhaps greater proof comes from the simple fact that, with the sole exceptions of psychopaths like Carl Panzram, everyone is pro-life to some degree.
In the end, pro-life is just that: pro life. This isn't a religious issue. You can be an atheist and agree it's wrong to kill. Indeed, you will notice the pro-abortion lobby often complains about gun rights because of their ignorant belief that banning guns can save lives. You see in particular Joe Biden, who supports abortion up until and including birth, currently complaining that an 18 year old can buy "assault weapons". So he's pro-life when it comes to teenagers.
And what about the argument that there's a difference between "I can't do that because of my religion" and "you can't do that because of my religion"? By that logic, you can't tell a Jihadi not to slaughter infidels because that's imposing your religion on him. But with the exception of the Jihadis themselves, most people agree that we should do what we can to prevent Jihadis from going on killing sprees.
This applies even to honor killings. The moral difference between abortion and honor killings is negligible. Both involve taking the lives of family members for selfish reasons. Why are honor killings illegal, and that's not considered a religious issue, but abortion is legal, and it is considered a religious issue?
To me, the reason is fairly simple. Those who claim to lack a religion do tend to be quite self interested. It makes sense that they, like all people, would react when other religions threaten their way of life, but be quite content when their own religion threatens others. But the value of life is a religious issue.
Every religion has its own take on which life, if any, is valuable, and how great that value is. If you claim not to have a religion, you get to make that call. If you want to pretend life is only valuable once it's left the womb (and in some cases, even then it's only valuable if the mother wants to keep it), so be it. But Christianity is unique in that it does not discriminate. Human life, regardless of age, race, gender, even religion, is equally valuable, because it is made in the image of God. For that reason, while there are scenarios in which it is morally acceptable to end a human life, these are few and far between.
And we tend to recognise that.
See, God does not make strange laws. From the moment we were conceived, we received a conscience. As we grow, we recognise the existence of this thing called "morality". We call some things "good" and other things "evil". We know there is a difference between peace and violence, greed and charity, pride and humility. Even if we only acknowledge it when it's convenient for us, we all know there is a difference between right and wrong, because that's just how God made us. Abortion isn't a religious issue for the same reason murder, theft, rape etc. are not religious issues. You don't get to throw out morality when you throw out God, and secular governments exist for this very reason. But you already know this.
Not only do you already know there is right and wrong, but you also know you don't always live up to it. Even lacking exposure to the Christian faith, you know you're not perfect. You know there's something wrong with the world, with your world. You make mistakes, you have often needed to apologise.
Abortion just happens to be one of the biggest mistakes a person can make. It's not just an "oops", like cussing in front of a kid. It's not just a filthy thing to do, like sending an unsolicited nude on Tinder. It's not even just a spur of the moment act of violence after losing one's temper. It's cold blooded murder of the most innocent and defenceless human beings on this planet. And this is so easy to prove that some people become pro-life in minutes, all because they have been shown a gore-free animation of exactly what happens during an abortion.
But to God, all sin is the same. Just as a single drop of urine taints the whole glass, so also is sin a direct affront to God. Abortion, cussing, God sees this all as so evil, He must pour out His wrath. But the good news is, the sinner doesn't have to take it. The Bible tells us "If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness." (1 John 1:9). If you are a woman who has had many abortions, or even if you are a doctor who has committed thousands, God still loves you enough to forgive you.
Beyond all imagination, He did so through the death of a single innocent. Jesus Christ, the one and only Son of God, was born as a human child to the virgin Mary. He lived a perfect life, completely free from any and all sin. Yet He died. Suffering the full wrath of God on a cross, Jesus died for your sin. Therefore, though guilty, you can receive the reward for His innocence. All who confess Him as Lord, and believe in their hearts God raised Him from the dead, will themselves be raised to life forever.
Abortion is not a religious issue. If you do not know God, you still have to be a decent human being, recognising that all others have value just as you do. But that will not save you. Only the God you currently deny can do that. If you continue to deny Him, you will be denied by Him. He will abort you, declaring "my Kingdom, my choice", and He will cast you off to the second death. But He has given you a better option: Repent, and be saved.