The morning bombing of the Church Bethel Injil in the city of Solo, in Central Java, was the latest in a spate of deadly attacks on minorities in the world's most populous Muslim-majority nation.
Kristanto said he and his wife were getting ready to leave at the end of the service when the blast went off.
"I was about to head home when a very loud explosion shocked me. A crowd of people from inside the church rushed to the streets," he told AFP.
"They were screaming and very hysterical. The peaceful Sunday has quickly become a chaotic situation."
"I helped several people who were injured and lying weak on the ground," said the badly shaken 53-year-old, who goes by one name.
Coordinating Minister for Political, Legal and Security Affairs Djoko Suyanto said two people were killed but refused to clarify whether the toll included the bomber.
"One died instantly at the site, the other died in a hospital," he said, adding that several other people were wounded, some badly.
The minister said President Susilo Bambang Yudhoyono strongly condemned "the act of terrorism."
"Nothing can justify this inhuman act," Suyanto told Elshinta radio. "It is the task of everybody to overcome this act of terrorism."
Solo, a city of 500,000, is the home of militant Islamist spiritual leader Abu Bakar Bashir, who was jailed in June for 15 years for funding a terrorist group that was planning attacks against Westerners and political leaders.
An AFP correspondent saw the apparent bomber's body on the ground at the church's main entrance. He was wearing a white shirt and black trousers and had his left hand severed.
At least 15 people were wounded and had been rushed to nearby hospitals, a local police official told AFP.
Most of Indonesia's 200 million Muslims are moderates, but the country has struggled to deal with a radical fringe of extremists who have carried out numerous attacks including the 2002 Bali bombings that killed 202 people.
Indonesia's constitution guarantees freedom of religion but rights groups say violence against minorities including Christians and the Ahmadiyah Islamic sect has escalated recently.
In June, a Muslim cleric was sentenced to one year in jail for inciting hundreds of people to burn churches and attack police.
Seventeen men were also jailed for up to five months for the February attacks on churches in the town of Temanggung, on Java.
Police have also been investigating a Good Friday plot to blow up a Jakarta church and a book bomb campaign targeting Muslim moderates and counter-terrorism officials.
No one was killed in those incidents.
Yudhoyono's government has faced growing criticism over its failure to respond to the spate of religious hate crimes.
Human rights groups also expressed outrage after a member of the Ahmadiyah sect, which are regarded as heretics by some conservative Muslims, was sentenced to six months for defending himself and others from a lynch mob which killed three of his friends.
The sentence was the same or stiffer than those handed out to 12 Islamic extremists who led the mob in the February rampage.
This happened an hour from where I live. Scary stuff. :\