Its Sins of a Solar Empire because its a cool title...
And most of us aren't such religion nuts that the word "Sins" is an issue all by itself. In fact, I don't think any but the most die hard fanatics would have trouble with the use of the word "Sins"... its not like your playing out sins, but rather dealing with the consequences of various Sins.
To elaborate a little further, 4x is typically used a descriptor for the complex turn based strategy games (Master of Orion series, Galactic Civilizations series, etc)