In addition to declaring himself a prophet, Ahmad told followers they did not have to pray five times a day, fast during the month of Ramadan or perform the pilgrimage to Mecca – the three pillars of Islam considered obligatory by most.
Muhammad Tubagus, Ahmad's lawyer, said he will appeal the verdict.
"Does the judge think he is God?" he said. "This is against the constitution."
Ahmad was arrested in October with six of his followers and charged in court after a group known as the Islamic Defenders Front called on the government to outlaw the sect.
Another influential Muslim group, the Council of Clerics, warned it would attack sect unless the government arrested its leaders.
Ahmad has since publicly retracted his claim to being a prophet.
Human rights activists and some other Muslim groups have accused the government of violating Ahmad's constitutional right to freedom of religion, saying it was caving in to pressure so as not to appear un-Islamic ahead of elections next year.
Last year the al-Qiyadah al-Islamiyah was declared as "deviant" by the country's top Muslim clerics, the same charge levelled against another Islamic sect, Ahmadiyah, this year.
Early this week thousands of Muslims joined protests in Jakarta demanding the government outlaw Ahmadiyah.
The government is preparing a decree to outlaw Ahmadiyah which claims to have branches worldwide, and which believes that Muhammad was not the last prophet.
Indonesia is the world's most populous Muslim country, with 85 per cent of its 230 million people professing to follow Islam.
In 2006, a former boxer was jailed for two years after leading ritual prayers in Indonesian, not Arabic as most Muslims believe is obligatory.
Last year, a female leader of a sect that blended elements of Islam and Christianity was also imprisoned.