So, how do you explain the Blessed Trinity to a Moslem?
And how do you tell a Moslem that Jesus Christ really did die on the Cross?
Before we do anything else we should pray for light from the Holy Spirit.
We can start to explain the Blessed Trinity to a Moslem by pointing out to him that when God speaks in the Koran, he speaks in the plural. He says "Nahnu" meaning we when He speaks in the first person. This same feature occurs in the Book of Genesis. God refers to Himself as "We" and "Us." This is a manifestation of the Holy Trinity.
Jesus died on the Cross because the Gospels tell us so.