Error 1. An exclusive focus on salvation - when theosis should be the main activity of mortal living
Traditional theology has tended to make salvation a problem, a difficulty, the proper main focus of our mortal lives. When actually salvation is as easy as wanting it and committing to follow Jesus through death into Heavenly life eternal.
This is clear from the Fourth Gospel (of 'John') - but traditionally Christianity chose to subordinate the Fourth Gospel into a framework provided by Matthew, Luke and Paul.
Salvation is in truth therefore a choice - and that choice is finally made after death; our nature and what we do in mortal life contributes to the outcome of that choice.
But salvation is not 'a problem', and attaining salvation is certainly not supposed to be the focus of Christian life. Each Christian is meant to have the happiness that comes from hope - and the hope which comes from faith - and to derive happiness and security from his decision to follow Jesus.
Christians are Not supposed to be fretting and worrying (or despairing) in doubts concerning the certainty of our salvation.
The proper focus of Christian life should therefore be 'theosis' or becoming more divine (more God-like) - more divine not temporarily in this mortal life; but eternally after resurrection.
In sum, this continued mortal life exists in order that we may become more God-like after resurrection: that is the reason of mortal life (and why we do not all die as soon as incarnated, while innocent - and before we have a chance to fall into damnation).
Error 2. Double-negative theology of mortal life - when it is actually an education
Traditional theology has sometimes included theosis, but only in a 'double-negative' context.
Original Sin is an example of double-negative theology. This posits that damnation (and Hell) is the default for all humans of whatever age, time or place - unless they are saved (by faith, the church, good works - or whatever). The negative is assumed, and Christianity posited to remove that negative.
For another example of negative theology; Medieval Roman Catholic theology and practice developed the idea that the post-mortal state of purgatory could be shortened and ameliorated by the activities of the living (prayer and other offerings). By the actions of mortal life, post-mortal life would be less-bad - which is what I mean by a double-negative conceptualization.
This is a negative theosis - we are not becoming better Men with a positive pay-off after death; not becoming more like God - but are alleviating various sufferings imposed by God.
The nature of mortal life was often conceptualized especially by Protestants in terms of salvation almost-purely (leaving out theosis) - so that by wrong choices in this mortal life would could fail to commit to following Jesus and be damned, or could sin and fail to repent and be damned...
At the extreme, our eternal state was dictated by our spiritual state at the moment of death - and the rest of life had no effect. Mortal Life was therefore a test; something that could be failed - but not something by which we could be spiritually and eternally improved.
Such a view of mortal life devalues it into a problem
But - especially after we have realized that salvation is not a problem - once we want it, and are prepared to do whatever is necessary to follow Jesus after death; then Mortal Life should be seen positively.
Our mortal life should be seen as potentially incremental and building towards a positive post-mortal outcome. We have experiences (provided for us by God, the creator) - and we may learn (what God intends) from these experiences - and thereby undergo theosis; and become better equipped for a more God-like life after death and resurrection.
Error 3. Regarding only Men as alive and conscious - when universal consciousness of beings is the reality
This error was already present at its foundation but has become much more severe over the history of Christianity. Until by now, Christianity is assumed to be only about men and women (Men) - human beings; and the rest of creation is assumed to be unconscious, lacking purpose - and most of the world is regarded as 'dead' - i.e. the 'material' world of physics which operates only by deterministic causes or 'randomness'.
This has led to the present usual idea of Christianity as being almost exclusively about 'morality'; as a moral play acted-out against a backdrop of dead and/or unconscious stuff.
But the truth is that this is a universe of Beings, who are all - in different ways - alive, conscious and with purpose. The drama of salvation and theosis plays-out for all Beings - in different ways.
As ancient Men knew, and as human children are born knowing; Man lives among Beings - and all that we know, we know about Beings.
When God created reality - God created Beings - and Beings are what God created - God did not create merely dead/ unconscious/ inert stuff
A corrected Christian theology
When these traditional errors are corrected we can realize firstly that - for those who have become Christian, and chosen to follow Jesus through death to resurrection as their primary commitment - this mortal life is mainly about theosis. Secondly that this theosis is positive - which means that the lessons we learn in this mortal life have a positive benefit for our post-mortal, Heavenly life. And thirdly that the drama of our mortal lives is enacted in a living and conscious world of purposive Beings - most of whom are not human.