We have to look into the notion of private confession in order to answer this question. Confession in ancient times, i.e., in the early Church, wasn’t organized the way it is now. Confession in the times of the apostles was public, not private: a believer confessed his sins in the presence of the whole community. This practice was explained first of all by the perception of the community as a Eucharistic entity, the “Body of Christ”, where every member was a member of that Body. Everything an individual did had its impact not only on his or her personal life but also on the life of the entire community. Therefore, sin was perceived not as something related to a single person but as a matter of concern for the church community as a whole.
It is hard to pinpoint the exact time when private confession appeared in the Church. We read that Hieromartyr Methodius of Olympus (3rd century) advised his spiritual children to disclose their sins to priests as often as possible so as not to have to confess publicly. This may mean that Christians were already aware of the practice of private confession that existed in parallel with public confession. With reference to that, there is a well-known principle coined by Saint Augustine who lived in the 5th century: the sins committed in private are to be confessed privately; the sins committed publicly need to be confessed publicly. The latter include robbery, theft, etc.
As far as keeping the contents of a confession in secret is concerned, here is what we should say. All the way until the reign of Peter I, confessions had always been kept secret. Disclosing the secret led to serious punishment of the offending priest.
However, the Spiritual Regulation of 1721 stipulated that priests were to collaborate with the police. Thus, according to the Spiritual Regulation, a priest was to inform the police about anyone who had committed or was planning to commit a crime against the government, e.g., treason, mutiny, attempt to assassinate the emperor or his family. It must be pointed out, though, that priests didn’t like being police informants.
Currently, according to the Basis of Social Concept of the Russian Orthodox Church (a foundational document that regulates inner and outer life of the Church), a priest is not allowed to disclose the contents of a confession even in order to help the law enforcement. There is only one exception: a priest may break the rule if the penitent shares information about a crime against humanity, such as a terrorist attack or obeying an unlawful order in wartime. With that said, a priest must try to talk that person out of committing the crime. If his attempt fails, he must warn those whose lives are in danger but not disclose the name of that individual. He is allowed to break the seal of confession altogether only in the case of an emergency.
Therefore, we see that a priest is not authorized to disclose the contents of a confession unless it may cause large-scale disasters, such as death of many people.
Translated by The Catalog of Good Deeds