It is crucial that Christianity is an opt-in religion - it must be chosen, it can only be chosen.
(Therefore Christianity absolutely entails the reality of agency, of 'free will' - and the impossibility of agency on the basis of mainstream modern metaphysics is a reason why normal public discourse is absolutely incompatible with Christianity.)
At the same time, Christianity is true.
This appears to set up some kind of paradox, in the sense that (surely?) if Christianity is true then it must be accepted; yet if it must be accepted then there is no real choosing of it...
My understanding is that this is indeed a genuine contradiction in mainstream 'classical' Christian metaphysics (in which God is omnipotent, and created everything from nothing) - a contradiction to which there is no rational answer; but not in a different theology. Because to deny Christianity on such a scheme, would be to deny reality - which is incoherent.
But, if we instead believe that creation is the effect of God shaping pre-existent chaos - including ourselves as God's children; then reality so far as it is ordered and understandable is God's creation.
However, the primordial chaos included beings: included God, and also ourselves (i.e. Men) but ourselves in a primordial, unconscious, disembodied sense - embryonic and lacking, but existing nonetheless. God's creation was the shaping of chaos, and the parenting of our primordial selves into God's children (as we are now, as we find ourselves).
All that is Good is inside this creation - creation is where the concept of Good has meaning. In particular all loving relationships are inside of creation, made possible by God's creation.
Yet there is another reality outside of creation; so denial of Christianity is not incoherent - there is another reality which might coherently be chosen in preference to God's creation.
What would such an opt-out of God's creation entail? Outside creation is not evil; but it is chaotic, meaningless, purposeless and lacking in any true relationships between beings.
Our primary choice is whether to opt-in to the reality of God's creation - or not. This is a real choice - and has real consequences. In principle a person might simply decline to join creation - and to surrender self-consciousness, and all the personhood which has been given us by becoming a child of God. This is not an evil choice - it is the choice of nihilism, of non-reality - but it is not evil (it indeed bears some relation to the ideal of 'Eastern' religions such as Hinduism and Buddhism).
The evil choice is to decline to joining God's work of creation; but to hold onto God's gifts to us - to hold-onto meaning, purpose and relationship - but to impose our own personal meanings upon them. It is to try and take what is personally gratifying from creation, but not to join creation. It is to adopt a stance towards creation that sees it primarily as a thing to be exploited.
In sum, Christianity is true - because it describes the world of God's creation, in which truth is given meaning and value; but this is not the whole of reality - therefore there is an alternative - therefore must opt one way or the other. And because we are agents (with free will) this choice is real and meaningful.
The necessity of opt-in arises because of the nature of God's plan for creation - which is one in which we (as Men) are agent and divine beings, in loving relationships, engaged in a mutual project of further creation.
(If creation were done and static, there would be no need for agency; but because creation is ongoing and endless, agency is of-the-essence.)
Among divine beings, there is no possibility of ultimate coercion - either we choose to join the great work of creation; or we opt-out fully - or else, as with evil entities, we try to exploit creation for personal gratification.
The work of creation ('Heaven') is both real and chosen.