When a Christian gets in a theological conversation with a Muslim, one of the first things that comes up is the mystery of the Holy Trinity, which we celebrate each year on this Trinity Sunday.
Muslims, like Jews and Christians, believe there is one God, all-powerful and transcendent.
Their concept of God resembles what appears in the Old Testament. This is understandable, because Mohammed, the founder of Islam who lived in the Middle East in the sixth and seventh centuries, grew up among Jews and Christians. But at that time, the Christian churches in the Middle East were constantly getting involved in theological controversies that caused violent and scandalous divisions among Christians.
This was the environment in which Mohammed adopted and popularized a simplified, non-Christian idea of God.
He rejected what Jesus had revealed about the Holy Trinity, that God is three divine persons - Father, Son, and Holy Spirit – but one divine nature.
That rejection certainly limited the room for theological controversy, but it also cancelled out the whole New Testament, which Muslims do not accept.
And so, of the world's monotheistic religions, only Christianity believes in the Trinity. It is unique to us.
It is true that the Trinity is hard to understand. How can God be both one and three? How can the Father, the Son, and the Holy Spirit be fully God, and yet distinct persons? Our minds cannot grasp this completely.
And yet, that very fact makes the doctrine of the Trinity ring true.
It shows that no merely human mind would have been able to come up with it. And it also shows that God, the Creator of the universe, exists in a way that we, mere creatures, cannot fully understand - and that makes perfect sense: God should exceed our ability to understand him; if he didn't, he wouldn't be much of a God.