Not all good is within Christianity - there is good outside it.
But, because we are (and this is just how things are, the way we find ourselves) children of God; because God made us what we are and made the world as it is - then that good outside of Christianity is a personal good. It is good for ourselves only.
What I mean is that we come to consciousness to find ourselves currently in a scheme or plan which comes from God. That we have consciousness comes from God. We are invited to regard this schemes as good, and to join with it - to accept and embrace God's plan of salvation and divinization of Men and Women.
To oppose the plan, as a matter of principle - to try and persuade others not to join in... well, that is evil, because we cannot offer anything better.
However, each of us, ourselves, as individuals, may reasonably decide we do not want to join-in with the plan - that we would prefer to opt-out. And this is not itself an evil decision - it may be a personal good - apparently the best conceivable course of action for us personally; it may be well motivated albeit selfish (in the sense of someone who knows what they he himself wants but not about others).
Because in the end, salvation and divinization are about happiness (in an elevated sense of that word) - and happiness is not a thing that can or should be forced upon a person.
So I am sure that God (as our loving Father) has made good provision for those who want to opt-out, who as individuals prefer not to join in with the plan. This provision has been termed Nirvana - an eternal (or as eternal as is desired) and blissful state of absence of consciousness.
Nirvana is not evil, to want Nirvana for oneself is not evil; but to preach the ultimate desirability of Nirvana is evil; not least because to do so is incoherent - given that the motive for choosing Nirvana is, and can only be, personal.