Looking at the contemporary scene, one may have doubts about Christians who seem to be united. The answer is that the present statement covers genuine Christians, observing and abiding. As for those who have moved away from their own religion turning into nonconformist, secular or atheistic individuals or groups, they are, for all practical purposes, out of the list of Christians even though, they may count them as Christians among the nationalities of a country. If, among such people, that religious dissension and mutual hostility does not exist, it would not be contrary to this verse because the conflict was based on religion, once the religion is not there, conflict too would not be there. As said earlier, this verse describes people who profess and practice the religion of Christianity and their division is well known.
Given below is a brief reference from Taysir in the marginal notes of commentator, al-Baydawi which reports that there were three sects among Christians originally:
1. Fisturyah (Nestorians)who took Sayyidna 'Isa (Jesus) as the son of God.
2. Ya'qubiyah (Jacobites) who believed in Sayyidna 'Isa (Jesus) himself as one with God.
3. Malka'iyah (Malkites) who believed in Sayyidna 'Isa (Jesus) as one of the Trinity.
And it goes without saying that in the presence of such major divisions in matters of belief, mutual malice is inevitable.
Verses 15 - 18
يَا أَهْلَ الْكِتَابِ قَدْ جَاءَكُمْ رَسُولُنَا يُبَيِّنُ لَكُمْ كَثِيرًا مِّمَّا كُنتُمْ تُخْفُونَ مِنَ الْكِتَابِ وَيَعْفُو عَن كَثِيرٍ ۚ قَدْ جَاءَكُم مِّنَ اللَّهِ نُورٌ وَكِتَابٌ مُّبِينٌ
يَهْدِي بِهِ اللَّهُ مَنِ اتَّبَعَ رِضْوَانَهُ سُبُلَ السَّلَامِ وَيُخْرِجُهُم مِّنَ الظُّلُمَاتِ إِلَى النُّورِ بِإِذْنِهِ وَيَهْدِيهِمْ إِلَىٰ صِرَاطٍ مُّسْتَقِيمٍ
لَّقَدْ كَفَرَ الَّذِينَ قَالُوا إِنَّ اللَّهَ هُوَ الْمَسِيحُ ابْنُ مَرْيَمَ ۚ قُلْ فَمَن يَمْلِكُ مِنَ اللَّهِ شَيْئًا إِنْ أَرَادَ أَن يُهْلِكَ الْمَسِيحَ ابْنَ مَرْيَمَ وَأُمَّهُ وَمَن فِي الْأَرْضِ جَمِيعًا ۗ وَلِلَّهِ مُلْكُ السَّمَاوَاتِ وَالْأَرْضِ وَمَا بَيْنَهُمَا ۚ يَخْلُقُ مَا يَشَاءُ ۚ وَاللَّهُ عَلَىٰ كُلِّ شَيْءٍ قَدِيرٌ
وَقَالَتِ الْيَهُودُ وَالنَّصَارَىٰ نَحْنُ أَبْنَاءُ اللَّهِ وَأَحِبَّاؤُهُ ۚ قُلْ فَلِمَ يُعَذِّبُكُم بِذُنُوبِكُم ۖ بَلْ أَنتُم بَشَرٌ مِّمَّنْ خَلَقَ ۚ يَغْفِرُ لِمَن يَشَاءُ وَيُعَذِّبُ مَن يَشَاءُ ۚ وَلِلَّهِ مُلْكُ السَّمَاوَاتِ وَالْأَرْضِ وَمَا بَيْنَهُمَا ۖ وَإِلَيْهِ الْمَصِيرُ
O people of the Book, there has come to you Our Messenger disclosing to you much of what you have been concealing of the Book, while he passes over much. 
There has come to you from Allah a Light and clear Book; with it Allah guides those who follow His pleasure in the pathways of peace, and brings them out, by His will, from the depths of darkness into the light, and guides them to a straight path. 
Certainly, gone infidel are those who say, "God is the Masih son of Maryam (Jesus son of Mary)." Say, "Who then has the power to do anything against Allah if He wills to eliminate the Masih son of Maryam and his mother and all those on earth?" And to Allah belongs the kingdom of the heavens and the earth and what lies between them. He creates what He wills. And Allah is powerful over everything. 
And the Jews and the Christians say, "We are the sons of Allah and His favourites." Say, "Why then would He punish you for your sins? In fact, you are just human, among those He created. He forgives whom He wills and punishes whom He wills." And to Allah belongs the kingdom of the heavens and the earth and what lies between them, and to Him is the return. 
Refuted here in this verse is only one saying of the Christians which is the belief of a sect among them that Sayyidna Masih عليه السلام is God. But, the argument employed in the refutation virtually covers the false beliefs of all sects against the inalienable principle of Tauhid, that is the Oneness of Allah. This may be a belief in a son of God or a belief in one of the three Gods the argument refutes all of them.
That Sayyidna Masih and his blessed mother have been mentioned here jointly could be because of two wise considerations. Firstly, it is the humility of Sayyidna Masih before Allah Ta'ala that he could never think of saving himself against the will of Allah, or his mother whose care and service a good son considers dearer than his own life. Secondly, by doing so, the view of the sect which takes Sayyidah Maryam as one of the three Gods also stands refuted.