Well, in ancient times, many if not most societies didn’t “see” infanticide as murder, either, with Egypt being a notable exception. The killing of very young children, whether passively or actively, was not only permissible but widespread and in some cases even encouraged. Likewise, it is hardly unknown for otherwise decent cultures have encouraged suicide by or euthanasia of the elderly and the disabled. At one period in Roman history, infanticide of deformed children was required by law! In other words, that some in our society do not “see” abortion as murder does not mean it was not in fact murder.
Besides, Catholics do not see the prohibition against murder, including the murder of an unborn infant, as something that depends upon appreciating the existence of a deity. It is a matter of natural law. No one would think a civilization that tolerates murder to be worthy of the name. The only question is when does human life (and therefore the right to life) start and under what circumstances is the right to life ever superseded by another compelling reason.
More to the point, let me ask you this: If it became the norm to “see” life as beginning in the sixth month after birth, which of us would not fight to make infanticide illegal even before the sixth month? None of us! We would not care that people thought us religious nuts!! In a democracy, especially, the blindness of others does not relieve the responsibility of those who see.