I assume that this question is referring to the most famous of Thomas Paine’s pamphlet series that is now called The American Crisis. If so, there are two reasons that Paine gives early in that pamphlet. One reason has to do with the virtues of the colonists and the other has to do with the evils of the British.
First, Paine says, God will favor the colonists because they did not want war. Instead, Paine says, the colonists tried to do everything they could to prevent things from getting to the point of war. He said that the colonists were people
who have so earnestly and so repeatedly sought to avoid the calamities of war, by every decent method which wisdom could invent.
Because they had tried so hard to prevent war from happening, Paine said, there was no way that God was going to allow them to be destroyed. In this way, the colonists’ virtues were a reason why God would favor them.
Second, Paine argues, the British clearly did not have God on their side. This was because they were not acting in ways that were just. Instead, he says, they acted so badly that might as well have been criminals. Paine compares the King of England to “a common murderer, a highwayman, or a house-breaker.” He says that the King has no more reason to expect God’s favor than any of those kinds of criminals. Therefore, the evils of the British also worked to make God favor the colonists.