5:73
لَّقَدْ كَفَرَ ٱلَّذِينَ قَالُوٓا۟ إِنَّ ٱللَّهَ ثَالِثُ ثَلَٰثَةٍۢ ۘ وَمَا مِنْ إِلَٰهٍ إِلَّآ إِلَٰهٌۭ وَٰحِدٌۭ ۚ وَإِن لَّمْ يَنتَهُوا۟ عَمَّا يَقُولُونَ لَيَمَسَّنَّ ٱلَّذِينَ كَفَرُوا۟ مِنْهُمْ عَذَابٌ أَلِيمٌ
They have certainly disbelieved who say, "Allah is the third of three." And there is no god except one God. And if they do not desist from what they are saying, there will surely afflict the disbelievers among them a painful punishment.
Translation · Saheeh International
Tafsir
Ibn Kathīr (abridged)
The Disbelief of the Christians; `Isa Only called to Tawhid
Allah states that the Christians such sects as Monarchite, Jacobite and Nestorite are disbelievers, those among them who say that `Isa is Allah. Allah is far holier than what they attribute to Him. They made this claim in spite of the fact that `Isa made it known that he was the servant of Allah and His Messenger. The first words that `Isa uttered when he was still a baby in the cradle were, "I am `Abdullah (the servant of Allah)." He did not say, "I am Allah," or, "I am the son of Allah." Rather, he said,
إِنِّى عَبْدُ اللَّهِ ءَاتَانِىَ الْكِتَـبَ وَجَعَلَنِى نَبِيّاً
(Verily, I am a servant of Allah, He has given me the Scripture and made me a Prophet.) until he said,
وَإِنَّ اللَّهَ رَبِّى وَرَبُّكُمْ فَاعْبُدُوهُ هَـذَا صِرَطٌ مُّسْتَقِيمٌ
("And verily Allah is my Lord and your Lord. So worship Him (Alone). That is the straight path.") He also proclaimed to them when he was a man, after he was sent as a Prophet, commanding them to worship his Lord and their Lord, alone without partners,