Which dogma states that? It seems to me that the Roman Catholic Church equates Ecumenical Councils (binding, infallible and inspired) on the same level as the Magisterium. If it is not a dogma then it is not binding. If it is a dogma, which Council made it a dogma necessary for salvation and equal to, if not higher than the belief in Trinity, Christology and Theotokos?
Participation in murder is a sin that cries out to heaven and damns the sinner for all eternity. The Orthodox do not think murder is sin?
But that's not what the Catechism says.
"2271 Since the first century the Church has affirmed the moral evil of every procured abortion. This teaching has not changed and remains unchangeable"
This is then patently false because, as mentioned earlier, the issue of ensoulment did not qualify all abortions as murder. Therefore every produced abortion could not have possibly been treated as murder. Even St. Augustine states that one cannot kill that which is not alive.
What is necessary for salvation is to avoid sin. To hold deathist views, or to vote for someone with such views is a grave sin of participating in murder.
The doctrines of the Church, whether consiliar or magisterial, or matter of natural law, are all binding. Further, matters of natural law, such as abhorrence of abortion and moral inacceptability of voting for the proaborts, is binding on everyone, not just on Catholics.
While the Church, acting on incomplete scientific knowledge, held an incorrect theory of late ensoulment, she always held abortion to be a moral evil, even if the late ensoulment hypothesis stood in the way of technically equating it with murder.