One of the greatest comforts I experience as a Christian is the unity of the Holy Scriptures. The Bible, despite it's having 40 or so authors, being written over multiple centuries, penned in different languages, still reflects a unified thought. It presents a holy God, dealing with sinful man graciously, and ensuring a route to salvation by grace through faith. In both Old and New Testaments man's right-standing with God depends on God's mercy and not man's efforts.
Genesis 15 lays down the principle of faith and the priority of a divine promise. When the Creator speaks, His creatures ought to believe. Try to understand Abraham and you will probably come the same conclusion as me: I've never been called to believe something more unbelievable in my own life. And yet, it's not the actual promise that holds weight. It's the One Who makes the promise- He is the one who matters. Whether God promised children to a barren couple or a child to perfectly healthy parents, what makes the saying trustworthy is the trustworthiness of the speaker. Yes, we ought to believe God.
In John 6:28-29, we find the following exchange,
Then they asked Him, "What must we do to do the works God requires?" Jesus answered, "The work of God is this: to believe in the one He has sent."
It's simply amazing. One would imagine that God's major issue with His people was their lack of obedience. Either they did what He commanded them not to do, or they failed to do what He commanded of them. For sure, God is always displeased with disobedience. Looking at the question and answer, however, we find that man's emphasis is not shared by God, Who has always maintained that His people's problem has been faith. Ultimately, disobedience is rooted in unbelief.
Notice how the question reveals the principle concern of man: "what shall we do..." The pride of fallen humanity is to do enough to earn one's keep. Sin-craving, self-serving man says, "If it is to be, it is up to me". But notice that Jesus' answer begins with the Prime Mover: "the work of God". What is required for man to be acceptable to his Maker? God must work on him. How is the man to believe in God? That is God's work, not the work of the believer. The religions of men begin with men, but relationship with God begins with God.
Finally, let's consider a few verses from Romans 4.
"Now, when a man works, his wages are not credited to him as a gift, but as an obligation. However, to the man who does not work but trusts God who justifies the wicked, his faith is credited as righteousness."
Here is the crux of the matter: God, by definition and due to His holiness, cannot be a debtor to sinful man. When men desire to do enough to be seen as worthy in God's sight, they have an aim that they would never overtly admit to: they want square and even dealings with the Immortal, Invisible, Only Wise God whereby they look Him eye-to-eye and demand rewards as a worker on payday. While that is blasphemous and impossible, it is amazing how God's glorious grace is able to penetrate sinful hearts and cause the seed of faith to grow and bear fruits of righteousness.
Moses is the one through whom God gave us the law. Jesus is the Son of God and the chief dispenser of God's grace through the cross and resurrection. Paul is among the chief interpreters/expounders of Jesus' teaching. The Bible accurately maintains their witness as a spectacular, brilliant, obvious testimony of the gospel-- a triple conjunction if you will-- that God justifies sinners by grace alone, through faith alone, in Christ alone, according to the scriptures alone, to the glory of God-- alone!