The definition of Mary as Theotokos (Bearer of God) is a guarantee of a correct understanding of the nature of our Lord Jesus Christ.
She was His mother, right?
He is God, right?
Then, ipso facto, she is the Mother of God. Or, do we have Nestorians and Arians amongst us who just cannot ascribe complete divinity and humanity to the Incarnate God?
Christos Voskrese!