History of Islam

Arab Muslim traders brought Islam to Indonesia possibly
as early as the 8^th century through Banda Aceh
However, Islam did not start to spread until the 13^th century.
Islam advanced as more of the archipelago
increased its trading with the outside world
The religion came into Java from ports in Banten and Batavia
By the end of the 16^th century
Islam had spread through all of Sumatra and Java. (Wikipedia)

Muslim Sects

Most Indonesian Muslims are Sunni
although there is a small minority of Shia Muslims.
There are also Sufi Muslims in Indonesia.
Many Sunni and Shia Muslims do not consider the Sufi to be true
Muslims because they use mysticism and believe
that they can have a relationship with God
Ahmadiyya make up another Muslim sect in Indonesia
Indonesian Muslim religious leaders do not recognize
Ahmadiyya as true Muslims but as a cult
however, there are 242 Ahmadiyya groups in Indonesia.

Folk Islam

Almost all Indonesian Muslims
whether Sunni or Shia, practice what is known as “folk Islam.”
Folk Islam mixes animistic practices with traditional Muslim rites.
For example, before a house can be built
a chicken must be sacrificed for protection.
The chicken’s head, feet, and wings are buried
under the foundation stones. 
The head must face east and be surrounded
by rice and spices.
The chicken’s blood is spread on the stones and
a coin is placed under the cornerstone.
Once this is done, the house can be built.
Yet, most Indonesians Muslims will also pray
five times a day and fast during Ramadan.

There are several Muslim organizations in Indonesia
that lead their followers in all areas of Islamic life.
The leading “modernist” organization is Muhammadiyah.
They have branches all over the country and have schools
orphanages
mosques, and clinics, along with many other public service projects.
Nahdlatul Ulama is the largest “traditionalist” Muslim organization.
It is also the largest Muslim organization in Indonesia
with 40 million followers.
They run the majority of Indonesia’s Islamic boarding schools.
There are also several other organizations
such as the Front Pembela Islam
a hard-line Islamic group that is known for violence.