An non-trinitarian could not, in all fairness, be considered Christian. They could be considered pseudo-Christian, like the Mormons, Jehovah’s Witnesses, Gnostics, and more recently, Oneness Pentacostals. While most Protestants would agree to the precepts contained in the Apostle’s Creed, they would argue that “I believe in the catholic church” means the invisible church of all believers, not the RCC. Even Anglicans and high-church Lutherans use the Nicene Creed, and the line “I believe in one, holy, catholic, and apostolic church” in effect means the same thing. However, there is only one Church that is one, holy, catholic, and apostolic, and that IS the RCC.
[quote=Contarini]You need to distinguish between agreeing with the content of the Creeds and actually using creeds. The more radical Protestants are anti-creedal in principle–they claim that Scripture is enough (although sometimes when various 18-19th-century Protestant groups criticize creeds they actually have the Reformation confessions in mind, not the ancient Creeds). But they would still believe in the doctrines of the Apostles’ Creed.
There are anti-Trinitarian Protestants, and Milton was one of them. However, most Protestants would say that anti-Trinitarians are not meaningfully part of the same category as they are–that is to say, most Protestants would claim to have far more in common with Catholics and Orthodox than with non-Trinitarians, and even anti-Catholic fundamentalists would say that Catholics are on the same level (i.e., that both groups are “not really Christian”). So in one sense labelling non-Trinitarians “Protestant” is a bit like labelling Mormons “Protestant”–true historically but in many ways very misleading. However, Protestantism has repeatedly produced a radical wing that denied the Trinity as un-Biblical. With this exception, the only parts of the Nicene Creed that Protestants might have problems with are the article on the Church (though Protestants would claim simply to reject the Catholic interpretation of that article but to accept the article itself) and the article on baptism (though Lutherans, Campbellite Restorationists, most Anglicans, and some Methodists accept the article without qualifications, and many other Protestants would say that they accept it in a conditional sense). Most Protestant denominations would claim to accept the doctrines of both creeds–even the Plymouth Brethren in Romania, who are as ultra-Protestant as you can get, make this claim.