well done for finding the church, i suggest you ignore this guy or only talk to him with someone else there with you as a 'chaperone'. he sounds like he has issues.
well done also for understanding some arabic
just remember never to say 'as salaam 'alaykum'!
the Christian version is 'salaam leekum' (non religious) or 'salaam bil Masih' (peace in the Messiah).
also people say 'ahlaan' (hi) or 'kayf haalak?' (how are you?) to which the response is 'alhamdu lilah' (praise God - exactly as the muslims say it).
Christians are all less likely to say 'subhan Allah' (glory to God) although they do say it sometimes.
to most, it sounds muslim. they would more readily say 'nashkor Allah' (we thank God)
these are the main religious differences in arabic greetings, oh and also Christians don't say 'bism illah' when eating, as this is short for 'in the name of God, the most beneficient, the most merciful' which is reserved for muslims. they rather say 'bism il Ab wa il Ibn wa il Roh il qadoos', 'in the name of the Father, the Son and the Holy Spirit.
(a few tips as a coptic Christian who is a very poor student of arabic who has muslim arabic speaking friends)