"And we are his house, if we hold on to our courage and the hope of which we boast." NIV translation
Now, Paul gives us the transition from the physical to the spiritual house. The Jews were used to the temple (house of God) but did not understand that God was no longer going to dwell in a building that was built by man. Jesus did not come and die to build up a fancy place for God to meet with His people but to open our hearts so that God can dwell within us.
The same type of thing is happening today as many jump from "church" to "church" looking for God without realizing that God wants to dwell within us 24/7 and not be put into some pretty building and visited once a week. Many have used this verse to say that we can lose our salvation as Paul said "if we hold on" but that is not what he was saying. He is reminding the Jews (and us) that we are a part of this spiritual house only because of what Jesus has done on our behalf. He was talking about the fact that there were many people that were looking for another way to be a part of this spiritual house. The same type of thing is happening today as there is a concentrated effort to deceive people into thinking that there is another way and that we are all going to get there just in different ways. My friends, that is a lie straight out of the depths of hell!
"So, as the Holy Spirit says: 'Today, if you hear his voice, do not harden your hearts as you did in the rebellion, during the time of testing in the desert, where your fathers tested and tried me and for forty years saw what I did. That is why I was angry with that generation, and I said, "Their hearts are always going astray, and they have not known my ways." So I declared on oath in my anger, "They shall never enter my rest."'" NIV translation
Paul reminds the Hebrews of the effects of doubt as their fathers had failed to believe that God had given them the land of Canaan. Because of this doubt, they wandered in the desert until that generation had perished. We, too, must believe that God has given us salvation through Jesus Christ and it is ours. If we fail to believe, we will wander this earth and then we will die without receiving what he has planned for us.
"See to it, brothers, that none of you has a sinful, unbelieving heart that turns away from the living God. But encourage one another daily, as long as it is called Today, so that none of you may be hardened by sin's deceitfulness." NIV translation
What did Paul mean by "turns away from the living God"? There are those who try to say that he is speaking of a believer losing their salvation but that is not what he was speaking of. Paul was talking about a person that has not accepted Jesus Christ as their Lord and Savior. They turn away (reject) God's way of salvation and, after a period of time, their heart is so hardened that they no longer hear the call of God upon their life.
Their Jewish fathers had rejected God's provision of the land of Canaan (they had not entered the land and then left it).