Is Christianity Monotheistic?

No!

Belief in Trinity (three entities in one Triune Godhead) straightaway opens up itself to polytheism.

The biggest dilemma of Christianity is mathematical :-) 1+1+1 = 1?

"Father, Son, Holy Ghost, each is a full God, yet they are one!"

Common sense will tell us that TRINITY cannot be Unity.

For nearly 2,000 years the best of scholars have utterly failed to satisfactorily explain the self-defeating dogma.