“For verily, Jesus did not set an example for you [Christians] of belief in the trinity or the opinion of intersecting spheres of existence [i.e. trilateral godship: the father, the son, the holy spirit]. Neither did he ever say that he is the Lord of all created things. Neither did he make permissible for you the flesh of the swine or other than it from impermissible [foods]. He never ordered you to leave circumcision or praying towards the east. He never ordered that you take your scholars and monks as gods besides Allāh [through taking legislation from them instead of the Gospel and Jesus]. He never ordered you to associate partners with Allāh, or to make idols and crosses, or to invoke the deceased or absent from the prophets and the righteous or other than them for your needs. He never [put forth the example] of monasticism or the other objectionable actions you [Christians] have invented. Truly, what you [Christians] practice is not taken from the messengers [disciples] of the Messiah.

Rather, the overwhelming majority of your practices are innovations, invented after the disciples. For example, your fasting fifty days in the spring [lent] and taking Thursdays, Fridays, and Saturdays as days of celebration. This was not the practice of the Messiah or anyone from among his disciples. Such is the case for Christmas, baptisms, and the rest of your celebrations.”

Source: Al-Jawāb al-Saḥīḥ 2:438
Translated by: Riyāḍ al-Kanadī