This would only cover the issue as far as a particular subset of Protestant Christianity, who subscribe to the Sola Scriptura doctrine. In the case of Catholic/Orthodox Christianity, the text of the Bible is **not **the sole source of dogma. Cunctator just made a reference that as far back as 1566 official Roman Catholic documents define abortion as a mortal sin. **Friar Ted **has referenced the *Didache *and a cite has been requested.
As mentioned, there seems to be a variation as to a pattern of legality v. illegality and social acceptability v. censure, varying much according to the particular culture, locally dominant version of Christianity, and even social divisiond of class or ethnic group. There have been peaks and valleys of looser and tighter norms. Through the history of “Christian” societies, it has NOT always been the case that all/only things considered “sinful” by the dominant church, automatically were both socially reprehensible and criminal in the eyes of society.
The question seems to be, not when did the churches determine it was sinful, but when did abortion (or FWIW, reproductive rights) become such a deal-breaking “litmus test” as a matter of a public policy (and public politics) position so that you don’t even bother waiting for an explanation. In the case of the Roman Catholic Church the big moment seems to come with the publishing fo the Encyclical letter Humanae Vitae, in 1968, which established that the RCC would take a hard-line position on reproductive rights/sexual morality, against ANY artificial hindrance of reproduction and against nonmarital, nonreproductive sexual conduct.
So, for some 400 years at least apparently it** IS **considered a serious sin, but like many other sins, something the Church dealt with mostly at the church/individual sinner level and maybe some stern sermonizing for the greater congregation. Then in the 1960s it becomes a major matter of public-policy positions. (The presumption is that this happens because by the 1960s, the medical science has progressed to the point where safe and effective contraception and abortion are potentially accessible to the population in general, and the social culture has moved in the direction of removing the social oprobium from nonmarital, nonreproductive sex, so with the two main drawbacks of physical risk and social censure removed from the regulation of sexual conduct, the Church is left to cranks up the position of how it’s morally “evil” to use these methods.)