I assume that you are asking about the first pamphlet in the series The Crisis. I make this assumption because it is the best-known of the pamphlets and because, in this pamphlet, Paine does discuss two major reasons why God would take the side of the colonists.
Paine’s first reason has to do with the virtues of the colonists. He says that the colonists have done everything that they possibly could to prevent a war from happening. Because they have tried so hard to avoid war, God will know that their intentions are good and will support them. As Paine says,
God Almighty will not give up a people to military destruction … who have so earnestly and so repeatedly sought to avoid the calamities of war…
Paine’s second reason has to do with the lack of virtue on the part of the British or, more specifically, on the part of the King. Paine does not think that the King is a good man. He believes that the King is so immoral that God could not possibly take the King’s side. Paine says that the King cannot look to God to take his side because he is no better than “a common murderer, a highwayman, or a house-breaker.”
In these ways, Paine argues that the colonists’ virtues and the King’s iniquity will cause God to take the side of the Americans in their struggle for independence.