Now he is a convicted heretic, banned from conducting services and hounded from his parish.
In spring 2022, Burdin denounced the conflict in a sermon to his parishioners and in comments online, saying Christians could not just stand by when a brother kills a brother.
He said the blood of Ukrainian civilians would be on the hands of Russia's rulers, its soldiers and everyone who approved of the war or simply kept silent.
As a consequence, he was fined for "discrediting the Russian army", and in June 2023 banned from conducting services by a Russian Orthodox Church court that convicted him of "heretical" pacifism and of undermining trust in Patriarch Kirill, head of the church and a close Kremlin ally.
But speaking to Reuters at an undisclosed location in Russia, Father Burdin said he does not see his preaching as anti-war, instead invoking the Biblical injunction against murder.
"From my perspective, it was a Christian sermon, not an anti-war one," he said. "We are all Christians, and we should not kill each other."
Any murderer, he said, "bears the sin of Cain", who killed his brother Abel in the Old Testament.
The Church hierarchy, strongly loyal to the Kremlin, has thrown its support behind Russia's war effort and ordered priests since September 2022 to recite a special prayer for victory during services.
Burdin is among dozens of priests punished for opposing the conflict, according to Christians against War, an online group uniting believers in Russia, Ukraine and Belarus.
In January, a church court said prominent liberal priest Aleksiy Uminsky should be "expelled from holy orders" for violating his priestly oath by refusing to read the victory prayer.
According to parishioners in Karabanovo, reaction to Father Burdin’s outspokenness on Ukraine was mixed.
Alexander, a church worker who did not give his surname, said some of the congregation had responded by asking Burdin to avoid politics, and focus instead on the spiritual lives of his parishioners.
One woman, he said, "got indignant and said you, Father, had better take care of the service, we do not need to be told about politics."
But Alexander said he continues to respect Burdin despite the ban, and would like to see him restored to his position in Karabanovo.
"He is a very good person for me. It never happened that I was hungry or had no clothes here. For me he is a normal and good man," he said.
Burdin said that after the controversy over his sermon, he had travelled to Bulgaria and contemplated becoming a priest there, a transfer he said was opposed by the Russian church authorities. But he said he had returned to Russia because he felt he was needed there.
Despite being banned, he said he continues to see himself as a priest and a servant of God. He said he had been struck by the words of his 12-year-old daughter, who told him the church could not prohibit anyone from continuing to serve God.
"I remain the same Father Ioann for all my parishioners who have known me as such," Burdin said. "I am not defrocked. I am simply banned from serving."