Can a believer be in unbelief? Yes, and unfortunately miss out on the promises that God has for them. The author of Hebrews defines "unbelief" as "not mixing faith with the Gospel" (what Jesus did at the cross), and gives us heed to a reality that should cause all of us to think about what we are trusting in, in our everyday "walk" with God. Check out all three teachings because in this, we will elaborate on the third chapter of Hebrews and see in the scriptures, what God says about unbelief. This will only lead to more freedom in your life as the cross is the ONLY Truth. The Truth that is continually setting us free, and bringing about the "promises of God" in our life.
Be Blessed!