Genocide is a particular thing. In order to classify as genocide, the attacker needs to be doing so with the explicit purpose of absolutely eliminating another identity of people. As far as I am aware, the US had no intention of killing off all the Japanese people in existence, but that the point was to send a message to them to stop attacking and attempting to conquer everyone else.
The Japanese Empire
If America wanted the Japanese people absolutely dead for the quality of being Japanese, then they would've continued to drop more and more atomic bombs until there was no more Japan, then they would've continued to kill any Japanese outside of Japan until there were no more Japanese in existence. This did not occur; once the Japanese surrendered, the killings stopped. It's evident that it was not an act of genocide, even if as a result a number of Japanese people died; nations attack other nations all the time, they don't necessarily do so because they want to genocide each other, it's usually because there's some other goal they're aiming for. I don't think anyone here is too naive to believe America was incapable of completely annihilating Japan if they really wanted to.
The second point: the United States may have a lot of Christians, but that someone is a Christian does not mean their behaviors are performed in the name of their religion. As far as I am aware, the US did not bomb Japan with the purpose of fulfilling their Christian faith. You could just as easily say, because the people who did this were white, then that means whites are responsible for this, then you could also say that because the people who did this were male, then that means males are responsible for this, then you could say the people involved mostly had dark brown or gray hair, so that means dark brown hair and gray hair people are responsible for this, furthermore most of them were taller than the rest of the population, so that means tall people are responsible for this, also you could say the people involved were well nourished, therefor people who aren't starving are responsible for this, also the people involved were alive in the 1900's, therefor people who lived in the 1900's were responsible for this, blah de blah etc. etc. etc.
Correlation does not imply causation, and not all members of these groups are responsible for the decisions of a few who happen to share some of their qualities.
if you can provide evidence that the people responsible for the two atomic bombs on Japan did so in the name of a Japanese genocide and that by a Japanese genocide they would be fulfilling their Christian faith, then you would have a point. Otherwise you're just practicing sophistry.