"He was faithful to the one who appointed him, just as Moses was faithful in all God's house" ( Hebrews 3:2 )
The writer of Hebrews is calling our attention to the covenants. He is drawing a parallel between the faithfulness of Jesus compared to the faithfulness of Moses. Both men ushered in God's covenants, Moses with the Law, Jesus with abundant grace. The book of John tells us, “For the law was given through Moses; grace and truth came through Jesus Christ" ( John 1:17 ). Both were faithful to God with what He had entrusted to them.
Since the original audience of the book of Hebrews was primarily Jewish, this statement would have been quite radical. Moses held a place of honor with the people far above any other prophet in the scriptures because he gave them the Law. Moses proved faithful before God, administering this covenant. In fact God said this about him in Numbers, “When a prophet of the LORD is among you, I reveal myself to him in visions, I speak to him in dreams. But this is not true of my servant Moses; he is faithful in all my house" ( Numbers 12:6-7 ). Even though the Israelites were hard-hearted, refusing to believe God, wandering the desert for forty years, this didn't nullify Moses’ faithfulness. He remains true to God and the covenant that was made.
Just like Moses, Jesus has remained faithful to the one who appointed Him Apostle and High Priest on our behalf. None of this bears weight on what we do, but everything depends on what He did. He has proven himself a faithful steward with what God entrusted to Him. And now Jesus resides permanently over the house of God administering this covenant of grace. In fact God has sworn an oath to Jesus that He will never change His mind, Jesus is a priest forever ( Hebrews 7:21-22 ). Because of this oath Jesus is the guarantee for a better covenant.
This is why the writer started chapter three reminding us to focus our thoughts on Jesus. Just like Moses, He has proven himself faithful. Understanding the covenant which He is the guarantor will bring us into a deeper revelation of grace. With this understanding, nothing will get in the way of the relationship that God has called us into.