One doesn't become a Christian as a means to the end of enjoying a happier mortal life here on earth. Christianity is true, and that is the reason why we ought to believe it.
The psychological (as well as the cosmic/supernatural) benefits of Christianity are available only to those for whom it is reality.
Yet, for someone who starts-out as a typical mainstream-modern atheist/ agnostic materialist - there are at least three immediate psychological benefits from being a Christian.
The background of mainstream modern materialism ("MMM") - which is explicitly taught us by public and official discourse in the mass media, education system, academia and science; and (especially) is assumed and implicit in the innumerable communications of state and corporate bureaucracy, law, and (above all) the mass media.
MMM has it that there is no meaning or purpose to reality - and the universe has nothing to do with me. Stuff happens in the world - or it doesn't - for reasons of deterministic-causality, or randomness. Things are not going-anywhere - except that material processes (mindlessly) unroll and interact.
For MMM; things do not add-up to any meaning that is relevant to me personally...
The universe cares nothing for me personally, neither does my individual consciousness and thinking have any effect on the universe - my mind is just a minor and insignificant part of a vast, impersonal, objective chaos of 'stuff happening'.
Starting from some version of the standard normal understanding of 'things in general'; Christianity offers the immediate benefits of understanding that I live in a divine creation which has a purpose and (therefore) meaning. It is going somewhere for real reasons.
When I think, when I am conscious; it is against this backdrop of an unfolding story - rather than a chaos going-nowhere. Furthermore, divine creation is a story that includes me specifically and personally.
Christianity (properly understood - i.e. rooted in Jesus) is always personal.
So that when I think, and what I am thinking, is relevant to the whole universe, the whole of creation. Creation is concerned with me, and my thinking and doing has an effect on creation.
For the new Christian; life is a drama in which he has a role to play.
As he becomes aware of himself and the world, as he becomes aware of his thinking - this personal and particular activity is known to be a significant part of the-whole-thing.
The third of these benefits - the personal aspect - is largely distinctive to Christianity among major religions; although there are, and always have been, self-identified Christians who down-play it to the point of obscurity*.
The new Christian has an immediate benefit of knowing not only that his life is part of a story linked with creation, but also that his life will not be (as with MMM) a brief and flickering flame lost in the unimaginable space and duration of the universe; but instead his own particular life will be be eternal.
Nihilism and despair followed by death are the usual consequence of mainstream modern materialism: Christianity is an immediate escape from this and into a life of coherence and consequence.
These three psychological benefits are an immediate outcome of becoming a Christian - or, at least, they are benefits for those people who desire that their own lives should have meaning, purpose and universal relevance...
Which is, apparently, not everybody - not by any means...
*e.g. By emphasizing the un-understandability, awesomeness, power, remoteness and abstraction of God - rather than the immediacy and know-ability and personal relatedness of Jesus and the Holy Ghost. Or by introducing weird and paralyzing philosophical concepts such as predestination, or that Time is an illusion - so that (somehow) everything in our lives has already happened. Such ideas obscure, when they do not negate, the essence of Christianity as personal. Of course; many/ most? people misunderstand Christianity and other religions - especially because they are unwilling or unable to consider the metaphysical assumptions and implications. So, more explicitly, what I am saying here is that Christianity - properly understood; is distinctive in its personal relevance to reality; when compared with other religions - properly understood.