In 325, Nicholas is said to have attended the First Council of Nicaea, where he is said to have been a staunch opponent of Arianism and devoted supporter of Trinitarianism, and one of the bishops who signed the Nicene Creed.
Nicholas's attendance at the Council of Nicaea is attested early by Theodore the Lector's list of attendees, which records him as the 151st attendee.
However, he is conspicuously never mentioned by Athanasius of Alexandria, the foremost defender of Trinitarianism at the Council, who knew all the notable bishops of the period, nor is he mentioned by the historian Eusebius, who was also present at the council.
Adam C. English notes that lists of the attendees at Nicaea vary considerably, with shorter lists only including roughly 200 names, but longer lists including around 300.
Saint Nicholas's name only appears on the longer lists, not the shorter ones.
Nicholas's name appears on a total of three early lists, one of which, Theodore the Lector's, is generally considered to be the most accurate.
According to Jona Lendering, there are two main possibilities:
Nicholas did not attend the Council of Nicaea, but someone at an early date was baffled that his name was not listed and so added him to the list.
Many scholars tend to favor this explanation.

Nicholas did attend the Council of Nicaea, but, at an early date, someone decided to remove his name from the list, apparently deciding that it was better if no one remembered he had been there.

A later legend, first attested in the fourteenth century, over 1,000 years after Nicholas's death, holds that, during the Council of Nicaea, Nicholas lost his temper and slapped "a certain Arian" across the face.
On account of this, Constantine revoked Nicholas's miter and pallium.
Stephen D. Greydanus concludes that, because of the story's late attestation, it "has no historical value."
Jona Lendering defends the historicity of the incident, arguing that, because it was embarrassing and reflects poorly on Nicholas's reputation, it is inexplicable why later hagiographers would have made it up.
Later versions of the legend embellish it, making the heretic Arius himself and having Nicholas punch him rather than merely slapping him with his open hand.
In these versions of the story, Nicholas is also imprisoned, but Christ and the Virgin Mary appear to him in his cell.
He tells them he is imprisoned "for loving you" and they free him from his chains and restore his vestments.
The scene of Nicholas slapping Arius is celebrated in Eastern Orthodox icons and episodes of Saint Nichola at Nicaea are shown in a series of paintings from the 1660s in the Basilica di San Nicola in Bari.
In 325, Nicholas is said to have attended the First Council of Nicaea, where he is said to have been a staunch opponent of Arianism and devoted supporter of Trinitarianism, and one of the bishops who signed the Nicene Creed. Nicholas's attendance at the Council of Nicaea is attested early by Theodore the Lector's list of attendees, which records him as the 151st attendee. However, he is conspicuously never mentioned by Athanasius of Alexandria, the foremost defender of Trinitarianism at the Council, who knew all the notable bishops of the period, nor is he mentioned by the historian Eusebius, who was also present at the council. Adam C. English notes that lists of the attendees at Nicaea vary considerably, with shorter lists only including roughly 200 names, but longer lists including around 300. Saint Nicholas's name only appears on the longer lists, not the shorter ones. Nicholas's name appears on a total of three early lists, one of which, Theodore the Lector's, is generally considered to be the most accurate. According to Jona Lendering, there are two main possibilities:
Nicholas did not attend the Council of Nicaea, but someone at an early date was baffled that his name was not listed and so added him to the list. Many scholars tend to favor this explanation.
Nicholas did attend the Council of Nicaea, but, at an early date, someone decided to remove his name from the list, apparently deciding that it was better if no one remembered he had been there.
A later legend, first attested in the fourteenth century, over 1,000 years after Nicholas's death, holds that, during the Council of Nicaea, Nicholas lost his temper and slapped "a certain Arian" across the face. On account of this, Constantine revoked Nicholas's miter and pallium. Stephen D. Greydanus concludes that, because of the story's late attestation, it "has no historical value." Jona Lendering defends the historicity of the incident, arguing that, because it was embarrassing and reflects poorly on Nicholas's reputation, it is inexplicable why later hagiographers would have made it up. Later versions of the legend embellish it, making the heretic Arius himself and having Nicholas punch him rather than merely slapping him with his open hand. In these versions of the story, Nicholas is also imprisoned, but Christ and the Virgin Mary appear to him in his cell. He tells them he is imprisoned "for loving you" and they free him from his chains and restore his vestments. The scene of Nicholas slapping Arius is celebrated in Eastern Orthodox icons and episodes of Saint Nichola at Nicaea are shown in a series of paintings from the 1660s in the Basilica di San Nicola in Bari.
Saint Nicholas