In 2000 a handful of troublemakers took over a peaceful protest and torched 10 churches in Mataram. The protest was to do with problems on Maluku where there was ongoing violence and atrocities being carried out by both Christians and Muslims against each other. A couple of tourist establishments were also threatened or torched. It lasted a couple of days but people have a long memory. There was much going on in Indonesia at the time, and there are all sorts of theories about what was behind the unrest on what is a laid back, relaxed island.
There is not a large local Christian population on Lombok, it is primarily Islam, followed by Hindu, followed by Chinese Buddhists. In Indonesia, Catholicism is a separate religion to Protestant, and of course they are both Christian. The Christians on Lombok are mostly immigrants from East Nusa Tengarra - the islands east of Lombok such as Sumba, Flores and Timor.
These are the churches that I have in my notebook: You should also check with your hotel as some hotels may have a service on Sundays, or be able to tell you were one is being held.
Protestant Churches:
Bethany Mataram Church
625-956
Jl. I Gusti Ketut Jelantik Gosa 23 Mataram
HKBP Mataram Church
632-924
Jl. Gili Air 4 Mataram
Kristen Tuhan Church
621-811
Jl. Ekas 47 Mataram
Masehi Advent H7 Church
638-500
Jl. Kom L Yos Sudarso 16 Mataram
Pantekosta Church
631-219
Jl. Pariwisata 4 Mataram
Catholic Church:
Katholik Church
634- 397
Jl. Majapahit 10 Mataram