He was not afraid to express rather radical theological opinions – controversial ones in some Christian ecclesiastical circles. He often pondered on the question of what influence his study of Islam had exerted on him in his own Christian faith. As a direct result, he came to argue that the Islamic emphasis on the uncompromising oneness of God had caused him to reconsider the Christian doctrine of the Trinity, which is vigorously attacked in the Koran as undermining true monotheism.
Influenced by Islam, with its 99 names of God, each expressing special attributes of God, Watt returned to the Latin word "persona" – which meant a "face" or "mask", and not "individual", as it now means in English – and he formulated the view that a true interpretation of Trinity would not signify that God comprises three individuals. For him, Trinity represents three different "faces" of the one and the same God.
https://en.wikipedia.org/wiki/W._Montgomery_Watt