In Mel Gibson's previous films concerning religion and faith there were always some elements included that established the existence of God beyond doubt. In the film "Signs" (2002) there were miraculous elements like Graham's dying wife had instructed him how to defeat an alien and then Bo's whim to put everywhere glasses with water had helped even further to defeat that alien. At the end of the film the director leaves no doubt that God exists regardless of whether characters believe it or not. The same relates to "The Passion of the Christ" (2004) film, except that the existence of God is established at the very beginning of the film.
In this regard the "Hacksaw Ridge" is very different. There are no miracles placed in the film, that could hint at the existence of God, except for the Desmond Doss' feat itself, which is interpreted as a miracle by his fellow soldiers, but the director seems to leave the question open as to whether it was a sign of God or an extraordinary human performance. Gibson never gives a definite answer to that question in the film.
And having left that open, he kind of proposed an idea that faith is a matter of choice. The characters in the film do not know for a fact that God exists. They choose to believe. The same way Gibson leaves the audience to choose for themselves if the feat was a sign of God, a miracle or not.
And then this idea of choice in reinforced by a story of Desmond Doss, who is shown to be a very religious man, who would rather go to jail than transgress a commandment. As the film progresses, first Desmond refused to even hold a rifle despite a threat of being sent to jail, then he chose to hold a rifle when he needed to save his commander's life. First he refused to work on a Saturday, because that would violate a commandment, but then he chose to violate this commandment in order to participate in a battle.
Gibson treats the commandments in the film not as an absolute obligation (the Law of God), but as something that can be chosen to obey or disobey depending on the circumstances.
So does this film push an idea that faith is a matter of choice and there is no knowing if God exists or not?