Unbelief is having a skeptical attitude; it is an attitude of doubt. People doubt many things, which there is nothing wrong with needing some questions answered. Doubt teaches one to research a matter, especially when one has questions about the supernatural and who God is. However, there is a problem when questions are answered without a shadow of doubt and unbelief remains. The problem may be a hardened heart.
God Speaks
God speaks to believers and non-believers. "All things were made by him; and without him was not any thing made that was made (John 1:3).​ "For ever since the world was created, people have seen the earth and sky. Through everything God made, they can clearly see his invisible qualities--his eternal power and divine nature. So they have no excuse for not knowing God (Romans 1:20), or believing in Him. We must believe in order to enter into God's rest.
There Remaineth A Rest
Unbelief because of a hardened heart will not make it into God's rest. "Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God. While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation. (Hebrews 3:12,15). Let us therefore fear, lest, a promise being left us of entering into his rest, any of you should seem to come short of it (Hebrews 4:1).
One Must Believe, Repent, And Have Faith In Order To Enter Into God's Rest