1John 2:11 But whoever hates his brother is in the darkness and walks around in the darkness; he does not know where he is going, because the darkness has blinded him.
1John 1:6 If we claim to have fellowship with him yet walk in the darkness, we lie and do not live by the truth.
If we have hate in our heart, we are "Walking around in darkness." If we claim to have fellowship with God while we are walking in darkness, "We lie and do not live by the truth." We are blinded spiritually, first of all, because we are in darkness, but also because our fellowship and ability to commune with God are impeded as long as we remain in a state of unforgiveness.
In order to come back into the light and into a state where we can hear God's voice clearly, we must forgive, as many times as necessary:
Luke 17:3 So watch yourselves.