Why is it so important that Christ was and is both God and man?

First, because otherwise Christ could not have accomplished the work of redemption. He had to become man in order to be able to die. And he had to be God in order to accomplish the work of redemption with divine power: “Having by himself purged our sins (Hebrews 1:3; compare Colossians 1:20). Furthermore, he had to be both God and man in order to be able to be the mediator between God and men (1 Timothy 2:5). A mediator is someone who can lay his hands on the shoulders of the people between whom he is mediating (see, by way of illustration, Job 9:33). Let us note that this is very important because it concerns the person of Christ. If a teacher comes to us, and does not bring “the doctrine of Christ”, we must reject him (2. John 9-11).