I was an Outsider more than thirty years before I became a Christian - having read Colin Wilson's book in 1978. Wilson himself never became a Christian, despite getting very close to it in his first two books (The Outsider of 1956 and Religion and the Rebel the year after).
Why should this be? - why should the inner-motivated Man, who regards himself as set-apart (for good, or more often for ill), perhaps having an unusual destiny, the 'existentialist'... why should such a person fail to recognise that his only satisfactory terminus lies in the truth of Christianity?
I think the reason is partly to do with the Outsider wishing to hold-onto his favourite vices (drink, drugs, promiscuous extra-marital sex and the like) but it is also the fault of Christianity - which has become identified exclusively with specific churches (with, quite often, each one stating that it uniquely holds the keys of salvation).
The Outsider sees Christianity as a choice of churches only. Now some Outsiders - such as GK Chesterton or TS Eliot - do find a home inside one of the established Christian churches (the Roman Catholic church being a favourite in the early 20th century). But they are clearly a minority.
There is insufficient awareness of the possibility of being Christian outside of any specific church. And/ or of becoming a Christian before, or without ever, joining a church.
For instance William Blake is an Outsider hero, and Blake was an absolutely devout and explicit and focused Christian in his Life and all-through his work. And (not 'but') Blake was a non-church Christian who was extremely unconventional/ heterodox/ heretical. Since Blake regarded himself as a solid and inspired and proselytising Christian outwith any church and with an unique set of convictions and practices; so too can any Outsider.
Furthermore, many churches conflate (link-inextricably-together) the possibility of believing in God, Christ and the immanence of God-in-all-things including ourselves (such as The Holy Ghost).
As that great non-church and heterodox Christian Rudolf Steiner said: to disbelieve in God is to be, in a real sense, insane; in other words, it is to disbelieve any possibility of coherence, meaning and purpose - which is to regard all of life as a delusion.
The reality and significance Christ is the only source of hope and ultimate happiness - all other religions are - if true, at their best and by their own account - miserable by comparison with Christianity.
And to deny God within us and the world is to live earthly life in a state of detachment - since we can only observe and never actually participate in reality: we can never know.
For an Outsider everything must, sooner or later, be tested by intuition in its widest and deepest sense; there must be a solid sense of personal conviction and relevance. With a church orientated Christianity, this is applied only to the question of whether a particular church is the one path to truth, reality and salvation.
Clearly, most Outsiders have the intuition that such claims are untrue, and therefore cannot and do not even wish to join a church for which they do not feel any such conviction.
But if existential conviction is the truest test, then it ought to be applied to sub-parts, and not merely to 'the whole package' as put forward by a specific church. Thus, an Outsider may be intuitively sure that there is a God who is creator.
He may additionally be sure that (in some vital sense) Jesus Christ is the Son of God and our saviour and central to our ultimate happiness (even though the exact meaning of the key terms is something he will need to strive to elucidate).
And the Outsider may also realise that his knowledge depends on there being something like the Holy Ghost - a divine spirit inside himself, and everybody else, and every-thing else - which makes possible true understanding and knowledge; and works over time to guide us to a more divine salvation.
Any Outsider who becomes a Christian is highly-likely to be heterodox, or regarded as heretical by many or most church members - but he ought not to be put-off by this: he should still become a Christian, simply because it is true (true in a real sense, albeit a sense that needs working-on).
Without Christianity, the Outsider is doomed to be merely a psychologist - since the most he can say in favour of anything is that it tends to make people happier... or at least to suffer less.
If the Outsider is to be able to use the concept of 'ought' then he needs to be a theist; and if he is to be someone who regards mortal life as important he needs to be Christian; and if he is to regard his own freedom and creativity as important, he needs to believe in the possibility of direct, unmediated contact with the divine.
What the Outsider gets from this kind of direct apprehension of the truth of Christianity; is great assistance in finding, sustaining and growing his true self - and then in discovering and pursuing his destiny.
He may well also become happier, more motivated and more confident in Life - but these are side-effects and never the primary aim.
In sum - the core reason for becoming a Christian is to convert an Outsider from being merely a Psychologist to becoming a real Prophet.