It is often assumed that opposition to abortion can be based only on religious premises. This assumption is plainly false. To show that it is is false, one need merely give an anti-abortion argument that does not invoke any religious tenet, for example:
1. Infanticide is morally wrong.
2. There is no morally relevant difference between abortion and
infancticide.
Therefore
3. Abortion is morally wrong.
Whether one accepts this argument or not, it clearly invokes no religious premise. It is therefore manifestly incorrect to say or imply that all opposition to abortion must be religiously-based. Theists and atheists alike could make use of the above argument.
And as a matter of fact there are pro-life atheists. Nat Hentoff is one. In The Infanticide Candidate for President, he takes Barack Obama to task:
But on abortion, Obama is an extremist. He has opposed the Supreme
Court decision that finally upheld the Partial-Birth Abortion Ban
Act against that form of infanticide. Most startlingly, for a
professed humanist, Obama — in the Illinois Senate – also voted
against the Born Alive Infant Protection Act. I have reported on
several of those cases when, before the abortion was completed, an
alive infant was suddenly in the room. It was disposed of as a
horrified nurse who was not necessarily pro-life followed the
doctors' orders to put the baby in a pail or otherwise get rid of
the child.