In the Christian theology, God is introduced based on the Trinitarian concept which is very much complicated to understand. The Qur’an considers Christians as the People of the Book not the Idolaters. It may mean that the trinity has a monotheistic origin.
From the Qur’anic perspective, God is not “the third of Three” as the Christians believe nor the Jesus Christ was son of God rather he was a servant of God. [1] Christians say that they do not believe in three separate gods, but rather in three qualities in a single Divine Essence and Trinitarian concept ultimately suggests a single Divine Essence. [2] Indeed, the belief is rather complicated to understand. Although, Christian’s reflection of God can be understood as Shirk, but the Qur’an never considers them as idolaters and calls them as the People of the Book. The Qur’an is full of God’s Names and Qualities which emphasize the Oneness of God: “He begets not; nor was He begotten. And none is like unto Him.”[3]
[1] Chapter 4, verse 171-172. يا أَهْلَ الْكِتَابِ لَا تَغْلُوا فِي دِينِكُمْ وَلَا تَقُولُوا عَلَى اللَّهِ إِلَّا الْحَقَّ إِنَّمَا الْمَسِيحُ عِيسَى ابْنُ مَرْيمَ رَسُولُ اللَّهِ وَكَلِمَتُهُ أَلْقَاهَا إِلَى مَرْيمَ وَرُوحٌ مِنْهُ فَآمِنُوا بِاللَّهِ وَرُسُلِهِ وَلَا تَقُولُوا ثَلَاثَةٌ انْتَهُوا خَيرًا لَكُمْ إِنَّمَا اللَّهُ إِلَهٌ وَاحِدٌ سُبْحَانَهُ أَنْ يكُونَ لَهُ وَلَدٌ لَهُ مَا فِي السَّمَاوَاتِ وَمَا فِي الْأَرْضِ وَكَفَى بِاللَّهِ وَكِيلًا
[2] The study of the Qur’an, Seyed Hussein Nasr (See the description of 4:171)
[3] Chapter 112, verse 3-4. وَلَمْ يكُنْ لَهُ كُفُوًا أَحَدٌ - لَمْ يلِدْ وَلَمْ يولَدْ