No, the Christian Gospel is not the only pathway to purification. There are others like Islam and Buddhism. Not all Christians believe Christ is the only way. But he is really good for those who have lost their way.
Regarding the choice of who to help first, I take the pragmatic approach and help whoever is hurting the most first.
I agree that Christian ethics are like ethics of the other main religions, and Humanism. I also agree that Christ is really good for those who have lost their way. This is why Nietzsche disliked Christianity, that it supports and perpetuates loss, death, and disempowerment, whereas it's better to empower lost persons so that they need no supernatural crutch.
The better way to be religious, and Christian, is to regard Christ as the good but helpless one that needs nurturing by men who are brave enough to carry the cross on his behalf.
-- Updated June 5th, 2017, 4:14 am to add the following --
(I omitted that the above quote was from JBH74)