Explaining the Trinity is a matter of explaining the unity and diversity of God. Typically, when Christians and Muslims talk about the Trinity, the Christian seeks to explain this unity and diversity, while the Muslim is critical, stressing that the unity and oneness of God makes any diversity impossible. At the popular level Muslim leaders stress that God only has unity and no diversity, and so this is the common Muslim assumption. This assumption needs to be challenged if you want to make progress explaining the Trinity. So how can we do this? How can we show that God has both unity and diversity?
We can do this by realizing that the doctrine of the Trinity is not the only doctrine of God’s unity and diversity. Consider Genesis chapter 1: Here we see unity, there is only one God, but we also see diversity as this one God sends his Spirit, acts through this word, and has many attributes that he expresses in a variety of ways. In Christian theology this aspect of God’s unity of being and diversity of attributes is examined in the doctrine of God’s simplicity. However, this is not only an issue for Christianity, every religion has some understanding of the unity and diversity of God - including Islam.
In Islam the question is, how do the many distinct attributes of God coexist in the unity of God’s essence and maintain his oneness. Islam maintains that each of God’s attributes (his power, knowledge, speech, life, will, sight, hearing, breath, etc.) are a distinct attribute yet all of them share fully in the one divine essence. How is this unity and diversity to be understood? The doctrine of Tawheed, with it focus on the oneness and singularity of God, is unable to explain this diversity as the following quote shows.
The Ash`aris maintain that the attributes of God are not the essence [dhat] nor are they other than His essence. If it is said that the attributes are the very essence of God (as the Mu`tazilah and philosophers claim), then it means that the essence of God is without attributes since they would be one and the same as the essence (whereas the attributes and essence are understood to be two different things). However, it is also problematic to say that the attributes of God are totally other than His essence, since it would mean that the attributes may exist separately and die away - whereas this is certainly not the case given that his attributes are eternal. The reality is that there is a special connection between His essence and attributes. His attributes exist in His essence, are eternal in His eternalness, and everlasting with His everlastingness. They have always been with Him and will be that way for eternity. (Muhammad Salih Farfur, The Beneficial Message & The Definitive Proof in the Study of Theology, (Trans: Wesam Charkawi) 2010, p. 119, underline added.)
Therefore Islam solves the problem of God’s unity of essence and diversity of attributes by saying there is a special connection between them. However, saying a special connection is not an explanation; it is just acknowledging that both unity and diversity are true and there must be some connection between them. For the Ash`aris this was expressed with the famous Arabic saying bi-la kayfa (without asking how).
Therefore, if Muslims do not accept the unity and diversity of the Trinity they still must acknowledge that in Islam God does have unity and diversity, and that Muslim leaders disagree about how to explain this. That is, Muslim leaders do not agree about Tawheed, and Muslims are rarely encouraged to think about the diversity of God. If you want to make progress with the Trinity you need to make Muslims think about this!
Therefore, if a Muslim asks you to explain the Trinity do not start with the Trinity, instead, first establish that God, even in Islam, has both unity and diversity. Do not let a Muslim fool you by focussing only on the unity of God. Ask them, “Are you saying God only has unity and no diversity?” Then you can discuss with them the following areas to establish that God has both unity and diversity:
As we have seen already, in Islam God has many distinct attributes each of which share fully in the one divine essence, that is, God has unity and diversity.
Did Allah create the Qur’an? The orthodox Muslim answer is no, the Qur’an is the speech of Allah and one of his essential attributes. Yet the Qur’an is distinct from Allah, therefore, there are at least two eternals, which means God’s unity is diverse.
Did Allah create his Spirit? The answer may vary from yes, no, and I don’t know. The Holy Spirit in the Qur’an is Allah’s breath through whom he gave Adam life (Q. 15:29). Allah sends his Spirit with revelation (Q. 16:102), and the Spirit comes as a man to Mary (Q. 19:17). Again, this shows a diversity in Allah.
Jesus creates a living being (Q. 3:49) in exactly the same way that Allah does (Q. 38:71-75). How can Jesus share in this unique God defining attribute and maintain the unity of God?
Islamic theology believes in a unity and diversity of God, therefore, Christians must not be fooled when Muslims try to discredit the Trinity by saying the unity of God makes any diversity impossible. After you have established that God has unity and diversity it is then time to ask what is the nature and scope of this diversity and how can we know this? Explain that we must allow God to reveal himself to us, and invite them to read a gospel.