In the Torah reading of Purim morning, we read about the initial battle between Amalek and the Israelites. We read about how when Moshe's hands were uplifted, the Jews would be victorious in battle and when his hands were down resting the Israelites would falter in battle.
The mishna and gemara ask about this how can it be that Moshe's hands were decisive in the success of the battle - what did his hands have to do with it? The gemara answers that it was not really Moshe's hands, rather they were the catalysts. When his hands were raised, the jews looked up. When they looked up, aside from seeing his hands, they saw the heavens beyond and remember Hashem and to daven to Hashem that they should be successful. When Moshe's hands were down, they continued to fight but had nothing to make them think about Hashem, so they would falter.
The emuna in Hashem that was increased at the moment is what effected victory, while the lack of emuna when they were not thinking of Hashem, rather possibly of their own strengths, is what effected failure in battle.
This is, in essence, the whole story of Purim as well. In the whole megilla we do not find Hashem's name. This is because the Purim story happened as a hidden miracle. That is also the symbolism of the name "Esther" which means hidden. her name was really Haddasa as the megilla tells us, but it refers to her as Esther because that was the essence of the miracles she was involved in while saving the Jewish people.
Because everything happened in a hidden way, it required pure emuna in Hashem. Without the emuna, Hashem might not have obliged and performed all these miracles. Without emuna, a person would have given up right away. Haman was very powerful - how can we fight against his decrees?
Only with emuna did they have the strength and conviction to go on and ensure salvation for the Jewish people.