Are Protestants Christians?

Once a person has reached the age of reason, yet still reject the Church, can that person still be considered a Christian?

What I meant was, once a Protestant reaches the age of reason.

I think St. Thomas said that the basic belief required to affirm it was the Trinity. But I found this section in the Summa dealing with Religion, interesting:

I answer that, as Isidore says (Etym. x), "according to Cicero, a man is said to be religious from 'religio,' because he often ponders over, and, as it were, reads again [relegit], the things which pertain to the worship of God," so that religion would seem to take its name from reading over those things which belong to Divine worship because we ought frequently to ponder over such things in our hearts, according to Prov. 3:6, "In all thy ways think on Him." According to Augustine (De Civ. Dei x, 3) it may also take its name from the fact that "we ought to seek God again, whom we had lost by our neglect" [*St. Augustine plays on the words 'reeligere,' i.e. to choose over again, and 'negligere,' to neglect or despise.]. Or again, religion may be derived from "religare" [to bind together], wherefore Augustine says (De Vera Relig. 55): "May religion bind us to the one Almighty God." However, whether religion take its name from frequent reading, or from a repeated choice of what has been lost through negligence, or from being a bond, it denotes properly a relation to God. For it is He to Whom we ought to be bound as to our unfailing principle; to Whom also our choice should be resolutely directed as to our last end; and Whom we lose when we neglect Him by sin, and should recover by believing in Him and confessing our faith.


http://www.sacred-texts.com/chr/aquinas/summa/sum337.htm

Being cut off from the Body of Christ, though, seems to suggest a person is no longer Christian.

This article just seems to express Marian Horvath’s opinion
in that it annoys her that it’s done. The Catholic Enccylopedia makes a distinction between calling someone a Christian and a Christian in the sense of the Catholic Church.

I don’t believe that the wide currency of the term to mean people who believe at least in the Trinitarian doctrine just occurred in the later half of the 20th century as Horvath suggests, but was in wider currency prior.

The truth of this is addressed by Trent and the presence of Protestants at the Council as well as the recognition that their baptisms were valid.

http://www.traditioninaction.org/religious/m013rpProtestantsChristians.html