First, I reject the view that the main fact of the human condition is an all-pervading entity desrcibed as 'sin'. In my experience, the main facts of the human condition are human goodness and human capability and these present themselves to us both when life is happy and sad, easy and difficult. It is not that sins of omission and of commission are absent from the scene; they are present. It is not that human foolishness is unheard of or unimportant; it is neither. But the fact remains that we are social beings and human society could not have survived for so long in so many variants unless human wickedness and folly, real though they are, are a minor rather than a major part of our human way of life. Society can only carry just so much human fallibility; the fact that the human race has survived and multiplied (criteria of success both biological and scriptural) suggests that we humans are not that bad and not that daft either. This general view is born out by the fact that my hundreds of varied acquaintances are, with a tiny minority of exceptions, people who contribute positively to the world they inhabit; I have no reason to doubt that I, and the hundreds I know, are untypical of the millions I don't know. Human worth is the big fact and the big reason for a rational faith that our species stands a very fair chance of rising above its problems in the real world of shared experience. Negatively, this is an atheist view but it it is better to describe one's position in positive terms - a cautious faith in shared humanity. That is Humanism.
Second, the idea that sin is not the dark all-encompassing blight of our lives makes the Christian recipe for salvation effectively redundant. Christianity offers a solution to a problem that isn't there to be solved. But the 'solution' is a most unconvincing one anyhow. The idea of a merciful god overseeing us scarcely fits the actual experience of innocent suffering and guilty escape. But, assuming for the sake of argument, that such a merciful god actually exists then I, quite unremarkably sinful man that I am, would feel able to face him in confident certainty of his mercy - I, and millions of others, have not done so very much to tax his capacity in that respect.
Third, I reject the peculiarly Christian idea that I need to be saved from the consequences of my sins, however great or small they might be, by suffering other than my own. Such an idea implies that I cannot take the can back for what I do and that the Christian god doesn't care whom he punishes so long as somebody suffers. That makes me a coward and god a monster and I'll have none of it - but I don't aim to kill those who think otherwise.