We are born as children of God, thus have within-us that which is divine.
As such we can recognise, we can know the Holy Ghost - which is divine.
Indeed, the Holy Ghost is Jesus Christ himself, which he sent after his ascension; as is expedient for us because the Holy Ghost is universal; available to all Men.
To believe in Jesus is the same as to trust, have faith in, and to love the Holy Ghost. (And vice versa.)
This, everyone and anyone can do - no matter what their personal circumstances.
Because all have a divine self, and all have access to the Holy Ghost.
And this is to be 'A Christian'. A real Christian.
(Even if when you have never heard of Christ; even if/ when Jesus is misidentified or misunderstood explicitly...)
Because what Men say, what Men teach is contingent and cannot be 'controlled' (even if God wished to control it).
What is needed must - in contrast - be certain, universal, immune to Men's wishes or abilities, and the uncertainties of circumstances.
What is needed must not depend upon prior knowledge.
And what is needed must be sufficient (even if it is not optimal).
Obviously so: Jesus loves us and is the creator and sustainer of this world; and he would surely not have left us with anything less?