I'm looking for a multilingual glossary of Christian/Bible terminology

Unfortunately this isn't going to be as easy as finding a good glossary. That's not to say that there aren't some good resources, but you're going to have to pull from several sources.

In the end when trying to do a good translation nothing will substitute for domain knowledge. In technical fields like engineering, a good glossary can be indispensable and a decent translator can get by translating outside of their own area of expertise just by knowing the language and having the right equivalents for technical terms. Softer sciences like philosophy and theology don't work quite that way.

In hard sciences the thing itself is usually fixed and hanging a term onto the thing gives everybody a handle to work from. When it comes to theology the terminology is often the most tangible thing we have to hang our ideas on and hence there are not always agreed upon terms for everything.

For historic language and contexts (such as Biblical Greek and Hebrew) this is somewhat mitigated by time, the amount of research brought to bear and the limited amount of material. Good lexicon's exist that can help you parse how a word might have been used in, for example, the Greek New Testament. These will help parse the word in a given context and list off other contexts for the same word and how it got used, helping to narrow down the range of semantic meanings an author at a given time in history might have meant. However they certainly aren't a translators panacea. Biblical authors often hijacked words to mean something specific in the context of what they were writing that might not have been familiar to even native speakers familiar with the word. Context is king here and hearing the author out in any nuances they build into a passage is the only way to pick an accurate translation into another language.

Moving into the realm of modern language, I would say there are two main categories of problems that make life rough on a translator.

1. Much of the terminology used inside in Christianity doesn't actually come from the Bible or some fixed authoritative reference source. Don't get me wrong, the ideas do (hopefully) stem from the Bible and are fixed, but the words we use don't necessary come from there.

   A classic example of this would be the concept of the Trinity (Üçlübirlik), one God existing in three persons. The concept of one single divine being God revealing himself as the Father, Son, and Holy Spirit is spelled out in the Bible, but the word "trinity" itself is an invention because people needed some shorter hand way to refer to the concept than rattling off the Athanasian Creed every time the subject came up.

   In general the Trinity is a pretty well defined term and even in Turkish you won't have a lot of trouble making it fit across translations. (I'm not saying the concept of the Trinity is easy, it's pretty much universally agreed that's a mystery we can't quite wrap our minds around, I'm only saying we happen to have an agreed upon handy term to refer to it.) You might run into trouble when you start describing the beliefs on non-Trinitarian groups. For example the heresy of modalism (God revealing him self in three different forms or as playing different roles rather than as three distinct persons) is easy enough to label in English (as are specific instances of it such as Sabellianism or Patripassianism) but you'll have a rough time when it comes to Turkish. The somewhat unoriginal import term modalizm exists but only people that have studied theology in a second language will know what it means. You have to explain it in context nearly every time (Bu üç kişisel öz, ne tek bir kişi tarafından oynanan üç ayrı roldür (yani modalizm), ne de bir küme içinde üç tanrıdır (yani triteizm)).

   That one is hard even though there is a very precisely defined meaning because either the words don't exist or people are always trying to explain it away as something simpler than it is.

2. Different traditions often use the same words to mean different things.

   Knowing and understanding the theological background of an author will make all the difference it accurately representing them in another language. Even very basic words like "faith" or "merit" are used in very different senses by Catholics than they are by Protestants. Without knowing that the fundamentals of how salvation is achieved are understood differently between these two traditions you might not pick up when those terms are used attached to wholly different concepts.

   Another good example of this might be "justification". While both Protestants and Catholics use the term, they imply a quite different set of events. While in a purely Protestant context using aklanma to translate justification is almost certainly your best bet for most contexts with an occasional use for doğru kılınma, I'm pretty sure that wouldn't work very well at all to describe Catholic theology.

   Other concepts will come with other problems. For example even inside of specific theological traditions such as Calvinism you will run into things like "limited atonement" (kısıtlı kefaret). Many people don't like this term. Largely the objection comes from other theological traditions where it doesn't fit inside their system and therefore seems to imply something they cannot accept, but even inside of Calvinism some authors prefer to use a different term emphasizing the positive aspect, for example "definite redemption" (belirli kurtuluş) is basically a synonym for limited atonement. Whether a given author actually means something different by it or is just avoiding the negative imagery is something you'll have to pick out from context.

In short you've got your work cut out for you if you want to do a quality translation of a theological into any language. For Turkish the situation is even worse. Christianity has a very old history in Turkey, but it's current presence is very small and only in it's infancy. As a result theological terminology in the Turkish language is not well rooted or widely agreed upon. Many terms just don't exist. It's like a bad joke, but Christians are regularly stuck making up new words as handles for established ideas that Turkish just doesn't have established terminology for. To add make matters worse an overwhelming percentage of books that have been translated into Turkish come from a very narrow theological spectrum: mostly Pentecostal / Charismatic. If you want to learn what words they use for things there are plenty of books to read, but it won't give you a very broad picture of Christian theology or terminology usage. Those traditions in particular aren't even big on using language in concrete ways to define their beliefs, often their own beliefs only have names because other people name them for them.

I already mentioned that not all theological terminology is found in the Bible, but that is at least one starting point. There are two full Bible and five New Testament only translations available (all available on incil.info). Unfortunately the most accurate translation (Kitab-ı Makaddes) uses very dated language that most Christians in Turkey today are not using. The more modern translation (Yeni Çeviri) is a much more dynamic equivalent translation and is rather inconsistent in its own usage, often translating the same term different ways in different contexts for no particular reason other than it sounded better. This makes it hard to discern how terms are actually used. As a translator trying to compare the Turkish to Greek or even English I would recommend Thomas Cosmades's translation for the New Testament. It doesn't flow very well from a literary aspect but he was much more selective in trying to use verbs that were a good match for the Greek. For the Old Testament you're just out of luck. Hit me up off-site and I'll hook you up with answers.

For general usage of terms that are not directly from the Bible, what you really need is a good Systematic Theology that covers a whole bunch of topics and what terms are connected to what. Sadly this doesn't exist yet in Turkish either. Tho closest thing out there is going to be J.I. Packer's Concise Theology (Teolojiye Giriş). This has 94 short chapters on various topics covering a pretty broad range of theological ideas. The author is an Anglican but covers theology from a Reformed Protestant perspective. In the book he does get into differences between different perspectives so there is quite a variety of terminology. Get in touch with Via Christus Yayınları for a copy.

_{Full disclosure: I'm directly involved in Via Christus and oversee both the Bible site and the publishing house linked above. My own theological biases tend towards the Reformed Presbyterian end of things.}