By Agence France-Presse
Alnoldy Bahari, 39, was found guilty of spreading hate speech for a post in which he claimed to have experienced God's presence and questioned the faith of other Muslims.
"He's been sentenced to five years' jail and has to pay a fine of 100 million rupiah ($7,200)," said Bahari's lawyer Andi Komara.
"We will most likely appeal because many facts weren't taken into account and were twisted."
The verdict will fuel fears that Indonesia's moderate brand of Islam is coming under threat from increasingly influential radicals.
Bahari, from the town of Pandeglang in West Java province, was charged with blasphemy and hate speech in December 2017, after a local leader of the militant Islamic Defenders Front (FPI) complained to police.
Although the blasphemy charge was subsequently dropped, Bahari found himself facing up to six years' jail for the loosely defined charge of hate speech, as defined under the electronic information law.
Demonstrators from the FPI -- many dressed in white and wearing skullcaps -- gathered outside the court as the verdict was read.
Rights groups have long campaigned against Indonesia's controversial defamation laws, which they say are unnecessarily vague and allow officials and wealthy individuals to criminalise critics and minorities.
"Indonesia needs an internet law that can protect rights to express opinions or even questions about religion," Damar Juniarto, Indonesia coordinator for digital rights group SAFEnet, told AFP.
"This Alnoldy case proves that people who only express their thoughts about a regular situation on Facebook... can be punished by the law."