Was Bahira the monk a monotheist or a Trinitarian, and was the Gospel in which he found the description of the Seal of Prophethood distorted?

The true nature of Bahira the monk or the Gospel he possessed is not definitively known, but it appears he was one of the Christian scholars. It is stated in "Dala'il al-Nubuwwah" (Signs of Prophethood) by Abu Nu'aym al-Isfahani that the Prophet, peace and blessings be upon him, traveled with his uncle Abu Talib in a caravan to Syria when he was ten years old. When they reached Busra, they dismounted near the monastery of Bahira the monk, where Christian scholars customarily inherited and studied books.

Bahira did not usually speak to them, but this time he prepared food for them and invited them, because he saw a cloud shading the Prophet, peace and blessings be upon him, and no one else, and he saw the branches of a tree bending over him.

Bahira noticed the absence of the Prophet, peace and blessings be upon him, and asked about him. When he appeared, Bahira observed him and inquired about his condition. When he looked into the Prophet's eyes and saw redness, he asked his companions about it, and they informed him that it never left him. Then he asked to remove his cloak, and he saw the Seal of Prophethood between his shoulders, which made his body shiver.

Then Bahira asked Abu Talib about his relationship with the Prophet, peace and blessings be upon him, and he informed him that he was his orphaned nephew. Bahira then advised Abu Talib to return with the Prophet, peace and blessings be upon him, to Mecca, and warned him against the Jews, saying that they would recognize his description from their books and would try to harm him, and that his nephew had a great destiny.

Some Jews did attempt to assassinate the Prophet, peace and blessings be upon him, but Bahira vehemently stopped them, asserting that they had no way to him. Abu Talib returned with the Prophet, peace and blessings be upon him, to Mecca, fearing for him. The Prophet, peace and blessings be upon him, remained protected by Allah from the evils of Jahiliyyah (the pre-Islamic era), until he was named Al-Amin (the Trustworthy).