Since a party or its leader a human, their authority is fallible and the foundations of the morality they create are questionable.
I don't dispute this - we should always be ready to question authority - but I do dispute your implied corollary - that divine moral authority is infallible. Even if we were to assume, for argument's sake, that God exists, where do I find his authority ? In a book written in the bronze age, by men, or in the pronouncements on that book by men (and some women) today. By your recognition, those guys are fallible, so there is no greater basis for religious authority over secular, in terms of morality - because the interpretation of religious authority necessarily filters through us unworthy humans. (And that, of course, is without even beginning to address which book and which God's or gods' priests to believe).
In the absence of religion a society derives its morality from another source (the only eg. i can think of is the state in communist countries)
Actually, what happens a lot in liberal democracies is that by some form of common consensus (the enactment of laws in an elected assembly, or the evolution of common laws in a democratic judiciary), morality evolves and becomes adopted into law by a wider society. It's not perfect, but it is a morality by broad consensus. On the whole, even given its imperfections, I like that a lot - certainly a whole lot more than abasing myself before some unknown, perfect entity, who reckons that I'm a fallen and pretty reprehensible being, whom he deigns to love, and purports to tell me what's right and what's wrong. (And who also told his followers to stone someone to death for picking up sticks on the Sabbath etc, which is stuff I reckon is rather hard to defend by any measure of morality).
In the absence of a god what reason do i have to behave according to anyone's so called morality?
The same as mine and everyone else's - your conscience; and failing that, the coppers. We live in a very secular society - and others in much more secular ones - and we don't see them descending into pits of immorality, lawlessness and evil.
Could there be any greater conceit, than for someone to believe that the universe has to be simple enough for them to be able to understand it ?