Aeterni Patris Filius

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Aeterni Patris Filius, meaning “Son of the Eternal Father,” was a papal bull issued by Pope Gregory XV on 15 November 1621 that set new rules for how a pope is chosen in a conclave. Together with the 1622 bull Decet Romanum pontificem, it formed the main framework for papal elections for most of the modern era.

Its goal was to reform the election process, establish clear rules, and reduce the influence of powerful factions within the College of Cardinals as well as limit the role of secular monarchs in choosing the pope.

Key points include:
- General rules for conducting the conclave were established, while Decet Romanum pontificem addressed ceremonial aspects.
- There were ideas from reformers like Robert Bellarmine and Federico Borromeo to end election by acclamation because it made vote counting and secrecy unclear; Gregory considered but did not fully adopt their proposals.
- Some radical ideas were proposed, such as holding the conclave in the presence of the deceased pope’s body or narrowing future votes to the top six candidates from the first round. Gregory rejected these as too extreme.
- A crucial reform was that a pope elected by scrutiny had to receive two-thirds of the votes in a secret ballot. This reduced the power of individual faction leaders and angered some Catholic monarchs.
- Acclamation was not abolished, but a secret ballot was made a prerequisite for elections by acclamation. Cardinals were required to take an oath to vote for a worthy candidate and to write their vote on paper that stated, “I choose as Supreme Pontiff my Lord Cardinal…” This aimed to prevent voting for friends or for jokes, and to reflect that the next pope would likely be a cardinal, even though non-cardinals could still be elected under the broad rules.

These reforms, together with Decet Romanum pontificem, remained the basis of papal elections until the 20th century, with only later changes introduced by Benedict XV and John Paul I. Some scholars now question how smoothly these reforms connected to earlier attempts dating back to Julius II’s time, but Aeterni Patris Filius is generally seen as the culmination of that longer process.