To be capable of becoming a Christian one must be capable of love. That's the basic necessity - and it seems most (but maybe not all) people are capable of love.
This is necessary because Heaven is a place of love - Heaven is a family; and it is love that gives Heaven cohesion and direction.
But Christian love is a personal love, a love between Beings, Christian love is Not abstract or diffuse, neither universal nor unspecific.
Those for whom love is universal do not want Heaven, but (maybe) something more like Nirvana.
However, love is not enough for a Christian; he must also want resurrected eternal life: he must want Heaven.
Some people do not want resurrection - but prefer oblivion (or even annihilation); while others want to become a 'pure spirit' rather than be resurrected with a body - and Christianity is not for them.
Life in Heaven is embodied, loving, personal - and essentially active, because personal love between incarnated Men leads to creation.
Or, more exactly, creativity is an aspect of personal love; it can be understood as the free activity of agents, capable of originating, generative, genuinely-novel thought and other behaviours.
Therefore those who instead want an eternal life of contemplative, passive, bliss - will not want Christian Heaven.
In sum, although in principle 'open to all', Christianity is not 'a universal religion'; because some Men lack the capacity for love or disvalue love, some do not want resurrection, and some do not want Heaven.
Heaven is for those who want to live in personal love, and to be active and creative - eternally.
In other words: 1. We need love in order to be capable of Heaven; 2. We need to regard love as primary to be a suitable candidate for Heaven; and 3. We also need to want the life of Heaven...
We need all three if we are actually to get-there.