Praise be to Allah.

Firstly: 

Believing in ‘Eesa (peace be upon him) as a Prophet and Messenger who was sent by Allah, may He be glorified and exalted, to the Children of Israel is part of believing in Allah and His Messengers. No one’s faith is valid unless he believes in all of the Messengers of Allah (blessings and peace of Allah be upon them). Allah, may He be exalted, says (interpretation of the meaning):

“The Messenger (Muhammad (blessings and peace of Allah be upon him)) believes in what has been sent down to him from his Lord, and (so do) the believers. Each one believes in Allah, His Angels, His Books, and His Messengers. They say, ‘We make no distinction between one another of His Messengers’”

[al-Baqarah 2:285]. 

Ibn Katheer (may Allah have mercy on him) said:

The believers believe that Allah is One, unique and Self-Sufficient; there is no god but He and no Lord other than Him. And they believe in all of the Prophets and Messengers, and the Books that were sent down from heaven to the slaves of Allah, the Messengers and Prophets. They do not differentiate between any of them, believing in some and disbelieving in others; rather in their view they are all truthful, righteous, rightly guided, and they guided people to the ways of goodness.

End quote from Tafseer Ibn Katheer (1/736).

As-Sa‘di (may Allah have mercy on him) said: 

Disbelieving in some of them is disbelieving in all of them; in fact it is disbelieving in Allah.

End quote from Tafseer as-Sa‘di (p. 120). 

Secondly: 

Celebrating the Mawlid or birthday of the Prophet (blessings and peace of Allah be upon him) is an innovation (bid‘ah). This was not done by the Prophet (blessings and peace of Allah be upon him) or by any of his Companions after him, and it is not known from any of the imams (leading scholars) of the Muslims that he regarded it as permissible or recommended, let alone participated in it. All of that comes under the heading of prohibited things and reprehensible innovations. 

The scholars of the Standing Committee said: 

Celebrating the occasion of the Prophet’s birthday is a prohibited innovation, because there is no evidence for that in the Book of Allah or in the Sunnah of the Messenger of Allah (blessings and peace of Allah be upon him). It was not done by any of his rightly-guided successors (the Rightly-Guided Caliphs) or the best generations.

End quote from Fataawa al-Lajnah ad-Daa’imah (2/244). 

Please see also the answers to questions no. 70317 and 13810 

What the common folk and ignorant among the Muslims do of celebrating the Prophet’s birthday is one of the innovated matters that should be opposed and prohibited. Referring to the celebration of the Prophet’s birthday as proof that it is permissible to celebrate the Gregorian New Year (or Christmas) is a notion that is based on false premises, because celebrating the Prophet’s birthday is not permissible, as it is an innovation that has been introduced into the religion, and whatever is compared to an innovation (for the purpose of justifying it) is also an innovation. 

Thirdly: 

The Christian celebration of what they call Christmas is an innovated and polytheistic (shirki) celebration. It is not permissible for the Muslims to imitate them in that, and ‘Eesa (Jesus – peace be upon him) has nothing to do with it or them. 

Rather for the Muslims – in addition to it being an innovation – it comes under the heading of imitating the disbelievers in matters that are unique to them and their religion. The Prophet (blessings and peace of Allah be upon him) said: “Whoever imitates a people is one of them.”

Narrated by Abu Dawood (3512); classed as saheeh by al-Albaani in Saheeh Sunan Abi Dawood. Its isnaad was classed as jayyid by Shaykh al-Islam Ibn Taymiyah, who said: 

The least that can be said about this hadith is that it implies that it is prohibited to imitate them. However the apparent meaning indicates that the one who imitates them is a disbeliever, as Allah, may He be exalted, says (interpretation of the meaning): “And if any amongst you takes them as Awliya’ friends, protectors, helpers, etc., then surely he is one of them” [al-Maa’idah 5:51].  End quote from Iqtida’ as-Siraat al-Mustaqeem (p. 82-83) 

Shaykh al-Islam also said: 

It has become clear to you that one of the causes of the diminishing of the religion of Allah and its teachings, and the prevalence of disbelief and sin, is imitation of the disbelievers, just as one of the foundations of all good is adhering to the ways and teachings of the Prophets. Hence the introduction of innovations into the religion is regarded as a serious matter, even if it does not involve imitating the disbelievers, so how about if the two matters are combined?!

End quote from Iqtida’ as-Siraat al-Mustaqeem (p. 116) 

Ibn ‘Uthaymeen (may Allah have mercy on him) said: 

Congratulating the disbelievers on the occasion of Christmas or any of their other religious festivals is haraam according to scholarly consensus, because it implies approval of what they follow of disbelief and approving of it for them. Even if he does not approve of this disbelief for himself, it is haraam for the Muslim to approve of rituals of disbelief or to congratulate someone else for them. Similarly, it is haraam for the Muslims to imitate the disbelievers by having parties on these occasions, or exchanging gifts, distributing sweets or plates of food, taking time off work, and so on, because the Prophet (blessings and peace of Allah be upon him) said: “Whoever imitates a people is one of them.” Narrated by Abu Dawood.

End quote from Majmoo‘ Fataawa wa Rasaa’il Ibn ‘Uthaymeen (3/45-46) 

For more information on the ruling on joining the disbelievers in their festivals, please see the answers to questions no. 1130 and 145950

To sum up: the harm that results from Muslims celebrating the Gregorian New Year (or Christmas) may be summarised in the following points: 

1.It involves imitating the mushrik disbelievers whose motivation in holding these parties is their shirk and disbelief in Allah the Almighty. It is not in accordance with the true teachings of the Prophet of Allah ‘Eesa (Jesus – peace be upon him), because according to our consensus and theirs he did not prescribe such celebrations for them. So they are a combination of shirk and innovation, in addition to what they mix with that of immoral and evil actions in these parties, as is well known. So how could we imitate them in that regard?

2.Celebration of the Prophet’s birthday is not permissible, because it is an innovation that has been introduced, as stated above. So it is not permissible to use it as an analogy, because if the basis of the analogy is corrupt, the entire analogy is corrupt.

3.Celebrating Christmas is an evil act in all circumstances, and we cannot say that it is permissible, because its very foundation is corrupt, because of what happens during it of disbelief, evildoing and sin. Such a thing is not fit to be used as an analogy for anything, and it cannot be said that it is permissible under any circumstances.

4.If we say that this corrupt analogy is sound, then we should be consistent and say: why do we not celebrate the birthday of every Prophet? Are they not all Prophets who were sent from Allah? And no one says this.

5.Knowing the exact date of the birth of any of the Prophets is not possible, even our Prophet (blessings and peace of Allah be upon him). The exact date of his birth is not known, as the historians differed concerning this matter, to the extent that there are nine or more different opinions. So the celebration of the Mawlid is invalid both in terms of the date and of sharee‘ah. Hence the matter in its entirety, whether it has to do with the birthday of our Prophet or the birthday of the Prophet of Allah ‘Eesa (Jesus – blessings and peace of Allah be upon him), has no basis at all.

Ibn ‘Uthaymeen (may Allah have mercy on him) said:

Celebrating the eve of the birth of the Prophet (blessings and peace of Allah be upon him and his family) is not valid, whether from the point of view of the date or the point of view of Islamic teachings.

End quote from Fataawa Noor ‘ala ad-Darb (19/45)

And Allah knows best.