The theme of Hebrews 3 is that Jesus is greater than Moses. The argument building up through Hebrews is that Jesus fulfils and therefore is greater than all of the Old Testament. He is greater than the prophets, greater than angels, greater than Moses and the Law.
In Hebrews 3, we see that Jesus is greater than Moses because whilst God used Moses as his servant to build a nation, a people of God, bringing them out of Egypt, bringing them to the land giving them the Law, Jesus is the Son and not just a servant.
Then the writer quotes from Psalm 95 including these words
“Today, if you hear his voice, do not harden your hearts as in the rebellion.”
Why is Psalm 95 included? Is it as simple as that the writer wants to quote a well-known passage that talks about Moses and the Exodus? I think it goes a little further than that. Let me explain. Psalm 95 is a description of how God’s people were provided for in the desert but rebelled against God, grumbling and complaining. This was not just a wone off event but frequent during their journey to the land culminating in their refusal to cross into the land after the 12 spies returned with their report.
Now, at first glance I might understand why the people were afraid to go into a land with strong fortresses and giants. That sounds quite fearsome. However, remember that God had defeated previous enemies such as the might of Egypt. Remember that God had brough them through the Red Sea, and how he had fed them. This enemy might seem fearsome but God has been with them in the past. They are not just hardening their hearts to God’s commands and promises but they are hardening their hearts to the testimony of what God has done for them in the past, they are re-writing their history.
Now notice that the writer talks about the Holy Spirit saying, “Don’t harden your hearts.” Flick forward in your Bible to chapter 5:11 where the writer talks about hearers who are spiritually dull. The idea here seems to be that they are at risk of becoming hard of hearing to the Holy Spirit. The next chapter goes on to warn about those who have experienced the work of the Spirit but then turn away. They are, he says, crucifying Jesus again and just like the Israelites in the wilderness, at risk of never entering God’s rest.
Again, the imagery is of people who have become hardened to their own experience. Sadly, this is all too real. I know of people who have experience of praying for people to be healed, expecting them to come to faith when healing happens but discovering that this is not the case. We all can no doubt think of examples of people who seemed passionate about the Gospel, or who were significantly helped in so many ways by the church. Yet those people are no-where to be seen today and have rewritten their own history to deny the goodness of those times. They have hardened their hearts to their own testimony.
I may not be in danger of doing things at that scale but I do think that I can easily forget what God has done for me in the past, how he has been there for me, the experience of his goodness, salvation and protection. Every time I try to take control, slip into legalism, allow snful habits to take over or become over-anxious, I act as though I have forgotten or even hardened myself to my past experience of Christ.
Moses in the wilderness for all his amazing gifts and his wonderful calling could not prevent the people from hardening their hearts, the Law could not help them. We need someone greater than Moses. That’s why we need Jesus.
 Psalm 95:7-8.
 Hebrews 3:7