I’m nitpicking (a little bit), but Christianity is not actually monotheistic. It says so in the Bible — the first commandment even: Thou shall have no other Gods before me. It is then backed up by the second commandment: Thou shalt not make unto thee any graven image.
Obviously Christianity is widely open to interpretation, based on the many distinct variations of faith, and I know at least Southern Baptists (personal experience) believe this is to mean that God is saying there is only one God — side note: a third proof is the use of phrase “one true God”, which infers false Gods (but Gods none the less).
But to my mind, and interpretation, this is God acknowledging that there are other Gods, but that he must come first and foremost in his worshipper’s eyes. Thus, Christianity never says there is just one God (monotheism).
/two-cents