It is Arabic and commonly used in Islam. Christians would not speak it normally unless they are non practising maybe. For sure a lot of Indonesians will understand it if you say it but then you will probably have to explain to them that you are not muslim as they will be excited thinking you might be. This is also very true for the phrases assalamualaikum and walaikumsallam. These are standard muslim greetings and if you use them then the next question from them will be, are you muslim or are you muallaf (converted to islam). For sure it is good to know what these phrases mean if you travel in this country but I don't know if it is a good idea to use them if you are not muslim, depends on the circumstance I guess. I will do a quick survey of my Indonesian colleagues and see what they say.
I guess it depends on area? Where are you that you have never heard it used?
I've definitely heard it used quite frequently, and I myself have used it a lot due to this. But I just wondered whether it was used at all in Bali, for example, or other areas where Muslims are not the majority. Or whether its usage has become more "mainstream" due to TV and media.
Ah, okay. Thanks for the insight. I haven't spent time in Jakarta, nor have I actually met any Indonesian Christians outside of the internet, so it's interesting to me which words are used more in different regions. Aside from the obvious dialects.
I've definitely heard "Astagfirullah" on TV and in Indonesian movies, by teen girls and headscarfed middle aged women alike lol