Plenty of people in the world - indeed, most people in the world, and virtually everyone who ever lived until the past several generations - believed in spirits; and indeed they believed in deity.
But, here and now things are more constrained. For one who believes in the reality and necessity of a spiritual destiny of Man and of each individual man and woman (you and me included) there is a necessary order in which we must attain knowledge; and this knowledge must be by personal revelation - by individual conviction.
Because, following on many decades of accelerating subversion and destruction of traditional, unreflective, 'automatic' spirituality - modern spirituality must be conscious, explicit and indeed personal, individual; addressed at the free agent which is our truest self.
So, in modern conditions we are inculcated with unbelief - and as a consequnce we are insane, lost and alienated; and this must be re-built, step-wise - since we cannot do everything all at once.
We must rebuild our fundamental, metaphysical assumptions - rebuild from foundations upward.
The foundation is to begin with deity, with God - that reality is is neither random nor are we merely a product of rigid causes... we each need to know that reality is created, hence has meaning and purpose. This must be the first revelation, our first personal conviction.
Then we need to know that this deity is God, that he is a person, that we are children of God - and therefore each of us may individually have direct knowledge of God (because we are like him; being offspring, we are of the same ultimate nature).
The revelation of Christ is necessary if we are to know that our future is one of meaningful purpose, genuine relationship and ultimate happiness; because it is Christ's gift to provide us with eternal life and the possibility of spiritual development after death.
(Without Christ, our fate is, at bottom, a bleak one - and this was easily understood 2000 years ago. The question then was whether the claims of Jesus were true - if they were true, then to 'believe in' then was 'a no brainer' assuming that happiness was wanted. Nowadays, largely because of atheism - that is, unbelief in deity and creation - people reject Christianity because it often interferes with optimising short term happiness in mortal life. However, we cannot believe in Christ by simple tradition and common sense - now we must have a solid, personal revelation of his truth.)
And only after the convictions of God and Christ are in-place, can we truly believe (that is actually live-by) the reality of the spiritual world - the Holy Ghost, as it were.
Here and now, our understanding of a revelation of the reality of the spiritual world can only be in the context or framework of God and Christ.
At this point - one is fully a Christian.
What then of a church?
Clearly, it is possible to be a Christian without believeing in the claims of any particular Church - but there are potentially (although not necessarily in practice) advantages to some Christian Churches - and it is likely that most Christians will at least explore the claims of the churches, of the various Christian denominations...
Each church asks different things before acknowledging belief - and many have different layers or levels of committment. But, since Christianity is ultimately a matter of the heart, the first step is again a personal revelation - that is, one may have a personal revelation of the truth of a particular church.
Only if this happens there is a further choice of whether to seek to join that church of which one has a personal revelation.
Thus - the process of being a Christian under modern conditions is much more individual and multi-layered than it used to be; it is much more conscious, explicit and a consequence of deliberate effort.
This is harder work and has more pitfalls than things used to be - but on the other hand, that is how things are - and we simply have-to work with it.
Also, such results are very solid at a personal level; and the nature of this kind of individually-validated faith is precisely what is required to be Christian in these end times or latter days; where we cannot any longer depend on social and institutional support.
(Rather; modern society and institutions - including most self-identified 'Christian' churches - are overwhelmingly against Goodness, against truth, and against us - and indeed, propagate an inverted various deadly brews of atheism, Christ-denial, and unspiritual materialism.)