1) Goodbye
Goodbye (also goodby, good bye, or good-bye), comes from the older English term godbwye, which is a contraction of God be with ye. (God be with ye = godbwye)
The French (adieu) and Spanish (adios) words for parting ways are also related to commending the person to God.
h/t Rich
2) Unicorn
We can thank the grumpy ole St. Jerome for this one.
In his 4th century translation of the Bible into Latin (known as the Vulgate), he translated the Greek word μονόκερως (monoceros), which literally means one horn, into Latin as the noun unicornus, based off the adjective unicornis, which means having one horn. The Greek term refers to the same animal as the Hebrew רֶאֵם (reem), which refers to a wild ox (not a one-horned horse).
When the translators of the King James Version came across the Hebrew word רֶאֵם (reem), they followed St. Jeromes literal translation of the Greek word, and used the English word unicorn. (E.g., see Numbers 23:22, Deuteronomy 33:17, Job 39:912, et al.) In the American Standard Version, unicorn was changed to wild ox.
3) Christen
Maybe this one is a bit too obvious?
Christen comes from the old English term cristnian, which means to baptize, or literally to make Christian. Today, the term can refer to the ceremonial launching of a new ship, or simply using something for the first time.
4) Infidel
This word is mostly associated with Islam today, but it was first a common word used by Christians.
It comes from the Latin word infidelis, meaning unbelieving, and meant non-Christian, sometimes more specifically referring to non-Christians seen as enemies of Christianity, usually Muslims (ironically enough!).
5) Charity
A charity is an organization that serves the community in some way, usually the poor. This is because these projects were originally expressions of Christian love, or charity.
Charity in the sense of love comes from the Latin term caritas, which St. Jerome used (most of the time) to translate the Greek word agape in his 4th century translation of the Bible into Latin, the Vulgate.
6) Limbo
To say that something is in a state of limbo means that its no longer making progress, is being neglected, or is in an undefined state.
This comes from the Latin term limbus meaning edge or border, which for centuries was used to refer to two regions on the outskirts, or edge, of hell. E.g., the limbus patrum, or the Limbo of the Patriarchs, is the place where holy souls prior to the coming of Christ waited after death until Christ opened heaven.
7) Fundamentalist
Being a fundamentalist is a decidedly bad thing today, although its precise meaning is unclear (too conservative? too literal? lacking nuance? close minded?).
But the term was originally coined by American Protestants in the early 20th century as a good thing, to denote the fact that they saw themselves as going back to the fundamentals of the Christian faith, in opposition to liberal Protestants who were demythologizing the faith.
#2 will make many Baptists I know lose their minds! (If it aint King James, it aint Bible!)