Why is Christianity considered a monotheistic religion?

Answer: there is one divinity in Christianity. Christian religion evolved from another monotheistic religion: Judaism. Christianity had to fight during all its existence with various forms of heresy. Some of Christian heresies are polytheistic, pantheistic, panentheistic etc.

Explanation: Of course Christian theologians could not escape an ancient metaphysical debate about ONE AND MANY (unity and multiplicity) as it existed a long time before. This debate was very serious and led to various church councils that - in the time of early Christianity - were supposed to define dogma. Ecumenical concil of Nicaea (325 AD) dealt with this issue of Christology (relation between Father-God and the Son, question of Holy Spirit was largelly not addressed). Relation between the tree or doctrine of Trinity was established later on in the 4th century AD (adoption can seem to be a "polytheist" element in Christianity but according to the dogma, the Son and the Spirit are of the same essence as the Father, i. e. they do not constitue a separated divinity).