Without actually seeing the original transcript verbatim, I can't be sure but a source that I saw this afternoon says that he said what I suspected, "bismillah raḥmani raḥim" which means "In the name of God, most Gracious, most Compassionate".
In Arabic, it's the phrase below if you want to paste it into Google Translate:
بِسْمِ اللهِ الرَّحْمٰنِ الرَّحِيْمِ
It's a phrase that begins most of the chapters of the Qur'an and it's something that you would hear a Muslim say before doing an act of faith. It also occurs in prayers and maybe the Christian equivalent would be a consecration like "Gloria Patri, et Filio, et Spiritui Sancto.."? It's something that Muslims memorize when they're children. Since Mateen's first language was English and his family language was Dari (and perhaps Pashto), I wouldn't be surprised if his Arabic were little more than what he memorized from prayers.
If he were to have said "Allahu Akbar" that would have made more sense. That would be an exhortation and something one says in excitement.
The problem is that when in English we say God in the singular we are referring to the God of Christianity and Judaism, ahich is quite different from Allah, notwithstanding the claims of Muslims.
Feel free to visit your local School of Divinity and share your insight and this unique interpretation of monotheism.
The teachings of God in the old testament are very different than Jesus in the new testament. I'm sure very similar to Allah and Mohammad. Christians today should follow the words of Christ and Muslims follow Mohammad.
Actually, no. In Christianity (except for some groups like Unitarians), Jesus is part of the Trinity, so he is God in the flesh.
In Islam, one of the basic pillars of faith is that there is only one God (the Shahada phrase is "There is no god but God. Muhammad is the messenger of God"). Mohammed is not worshipped nor followed since he not a deity (which is why likenesses of Mohammed are never created). Mohammed is more like Moses in Judaism and Christianity- a prophet and a leader, since he founded Islam, united the people and was given revelations that were written down and became the Qur'an.
In Islam, Jesus (known in Islam as Issa) is considered a prophet, too, but not a supernatural being.
The point is that these Abrahamic religions all originated with the belief in one God. That belief was revolutionary but also has been the source of a lot of conflict with other religions. As time goes on, the inherent and historical conflicts between groups of people has led to conflicts between Jews, Christians and Muslims... and intrafaith conflicts between Protestants vs Catholics, Sunni vs Shi'a, Reform Jews vs Orthodox... in spite of their common God.