I have 2 questions from a christian brother.

First one i told him we dont believe in the trinity and explained its flaws to the best of my ability. So then he asked me how to properly justify that Allah (swt) is 1 and not in 3 as they say. What is an answer to this?

My second question is how does Allah (swt) choose the prophets in a fair and just way?

1) If God is One Essence but in three different forms or parts than all of these forms would have to have been Qadeem (eternal). However, we see that Jesus (as) which is the 2nd part of this trinity of Father, Son and Holy Spirit was himself created and born of the virgin Mary (as). Secondly, If God were made up of three forms then it would mean that God would be in need of His parts to be complete. Thirdly, if all of the attributes of Father, Son and Holy Spirit are separate from His essence than this would mean we would need to have some gap to differentiate between them and this would mean that all of them would be in need of these gaps and God is free of need from anything, thus it is impossible for God to have any partner.

‎يَا أَهْلَ الْكِتَابِ لَا تَغْلُوا فِي دِينِكُمْ وَلَا تَقُولُوا عَلَى اللَّـهِ إِلَّا الْحَقَّ إِنَّمَا الْمَسِيحُ عِيسَى ابْنُ مَرْيَمَ رَسُولُ اللَّـهِ وَكَلِمَتُهُ أَلْقَاهَا إِلَى مَرْيَمَ وَرُوحٌ مِّنْهُ فَآمِنُوا بِاللَّـهِ وَرُسُلِهِ وَلَا تقُولُوا ثَلَاثَةٌ انتَهُوا خَيْرًا لَّكُمْ إِنَّمَا اللَّـهُ إِلَـهٌ وَاحِدٌ سُبْحَانَهُ أَن يَكُونَ لَهُ وَلَدٌ لَّهُ مَا فِي السَّمَاوَاتِ وَمَا فِي الْأَرْضِ وَكَفَى بِاللَّـهِ وَكِيلًا

“People of the Book, do not exceed the limits of devotion in your religion or say anything about God which is not the Truth. Jesus, son of Mary, is only a Messenger of God, His Word, and a spirit from Him whom He conveyed to Mary. So, have faith in God and His Messengers. Do not say that there are three gods. It is better for you to stop believing in the Trinity. There is only One God. He is too glorious to give birth to a son. To God belongs all that is in the heavens and the earth. God alone is a Sufficient Guardian for all.” Quran 4:171

Another point to mention is that we know Allah is not in need of anything and has no deficiencies. Humans sleep because they become weak and they need sleep to recharge. We can all admit that Jesus (as) slept. How can we say a person who has slept is all powerful? We all can say that Jesus ate food and drank water and this is done in order to gain energy. If we go without food or drink we will collapse. This is a weakness that man has but Allah has no weaknesses. And lastly after we eat the food is processed through out body and we relieve ourselves by going to the restroom. How can we say that anyone who does something as filthy as using the restroom is Allah?! This goes against out intellect to accept such a thing.

2) Allah sends us his perfect message and chooses the best to bring us that message and be an example for mankind. He knows best where to place His message as He knows His creation the best.