God told Israel the promised land was theirs. Yet it was possessed by others some were even giants which made it look impossible for them to possess. God had given them the land, but they had to go in and take it.
God would enable them to take possession, if they would only cooperate by believing Him. Yet they focused on the seemingly impossible circumstances and took their eyes off God's promise. Only one thing kept the people of Israel from taking possession of all their promised land unbelief! Maybe they felt incapable, but that was the devil's lie. Their capability was God! God had spoken! But they chose not to believe what God said to them.
Today it is no different with us! The New Testament reveals many things which already belong to us because we are joined with J Christ. These are not just promises, but statements of what is already ours because of
God's wonderful grace. God has given us richly all things to enjoy (1 Timothy 6:17, & 2 Peter 1:3). Many of us are not enjoying much, but it is NOT God's fault. We are the ones who need to change. We need to change what we believe! Many expect Bible promises to just fall on them. But you have a part to play. Nothing automatically manifests in your life until you believe and act on God's Word. You must actively receive what the Bible says is yours. Very often in the New Testament the word "receive" translates the word "lambano," which means to actively receive. This word is often translated as "take." We must take possession of what God has given you. Without actively taking what belongs to you, you will not enjoy all God has provided for you. Remember that faith is the noun form and believe is the verb form of the same word in the New Testament. So if our beliefs don't line up with God's Word, then we need to change our beliefs to be line with the perfect will of God, for God's Word is truth. If we get line with his Word his promises for us become real, otherwise they just sound like pipe dreams.
So we see that they could not enter in because of unbelief (Hebrews 3:19).