Ethics consists in subordinating the short term to the long term. The longest-possible term is eternity. A doctrine that makes you think in terms of eternity provides the greatest-possible frame for ethical thought and behaviour.
An ethical doctrine that includes eternity will maximise ethical effects, provided that it teaches to do good. If it teaches to do bad, like Islam, it does the greatest opposite: it maximises evil thinking and doing. Christianity is an ethical doctrine that includes eternity and teaches to do good.
Ergo, Christianity maximises ethical thinking and doing, compared with both Islam and atheism or agnosticism.