Monotheistic religions are generally authoritarian, prescribing rules and beliefs from which their members can not dissent. In most cases, leaders are chosen by an inner circle and imposed on members, who no doubt willingly accept their religious leadership in most cases. Democracy, on the other hand provides its members the opportunity to seek change to the rules at the ballot box. Members of a democracy choose their leaders and can change them by majority decision.
Ideals that monotheistic religions share with polytheistic religions and even atheism do find common ground in democracy. Most religions would, for example, seek to alleviate the suffering of the less fortunate in society, which is also an objective of most democracies. The ideals of charity and unselfishness are almost universal and are therefore held, at least formally by all religions including those that are monotheistic, and underpin the concept of democracy.