May 20, AD 325

First Council of Nicaea

Traditional opening date of the First Council of Nicaea

Constantine came in without his usual soldiers, stood before a low golden chair, and waited until the bishops beckoned him to sit.

The opening session at Nicaea

On the traditional — but disputed — opening date of 20 May 325, bishops assembled in an imperial palace hall at Nicaea in Bithynia. Eusebius of Caesarea, a participant, counted more than 250 bishops, besides presbyters, deacons, acolytes and attendants he said were beyond counting.

Constantine entered ceremonially, waited for the bishops to invite him to sit, and took a low golden seat among them. He then delivered an appeal for ecclesiastical peace in Latin, through an interpreter, to an overwhelmingly Greek-speaking assembly, and left the presiding bishops to begin the debate.

What 20 May is, and is not

The year is secure; the day is not. Ancient evidence does not establish an uncontested opening date — 20 May is the traditional reconstruction, and dates in June have also been proposed.

Eusebius describes an appointed imperial session but does not attach 20 May to it, so preliminary meetings may have preceded Constantine’s ceremonial appearance. The summons had gone out earlier; it was not issued that day.

Everything famous came later in the council

This date marks the opening, not the same-day adoption of the creed, the Easter settlement, the disciplinary canons or the penalties against Arius. Those came during the council that followed.

The council’s detailed acts have not survived. The principal surviving conciliar documents are the creed, twenty canons and a synodal letter — and that 325 creed, dense with clauses about the Son, disposes of the Holy Spirit in a single short phrase.

Sources

Researched 23 Aug 2026 6 sources 2 audit passes, 1 issue open Traditional date

How this was checked

Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →

Traditional date. This date is the conventional one, not one fixed by a contemporary record.

What the sources leave uncertain

The year 325 is secure, but ancient evidence does not establish an uncontested opening day. May 20 is the traditional reconstruction; dates in June have also been proposed.
Eusebius describes an appointed imperial session but does not attach May 20 to it, so preliminary meetings may have preceded Constantine's ceremonial appearance.
The council's detailed acts have not survived; the principal surviving conciliar documents are the creed, twenty canons, and synodal letter.