Although God never changes, He specifically changed in the way in which He dealt with mankind. One way is called the Old Covenant under Law and the other is called the New Covenant under Grace. The Old covenant kills, that was its ministry.
Now if the ministry that brought death, which was engraved in letters on stone, came with glory, so that the Israelites could not look steadily at the face of Moses because of its glory, fading though it was, will not the ministry of the Spirit be even more glorious? 9If the ministry that condemns men is glorious, how much more glorious is the ministry that brings righteousness!
(2 Corinthians 3:7-9)
Notice the Old Covenant brought 'death' but the New Covenant brings righteousness. There was always grace in the Old Testament and there is Law in the new, however, it’s simply a fact that God first more fully manifested His wrath for sin under law and then revealed His infinite grace and love in His Son on the cross. God want to terrify all men in their sin, so that they run to Christ for forgiveness and find his everlasting love. We are all born into the Old Covenant and God want us to enter His love by faith. Furthermore, God became a literal citizen of Israel in the old order of things, so that Israel personally executed his wrath for sin on sinners. God’s kingdom was visible, so when the Hebrew sword cut down the sinner, this was nothing less then a violent entrance into hell under God’s wrath and law. In the new covenant his grace has been revealed to save men from that very wrath.
If you work this idea outward, the result is that the very different moral impressions that one might in their confusion draw, when looking at the Bible on its surface, faith finds as a source of joy. This difference in the covenants is a confirmation of faith for it explains both death under sin and life under grace.
For the law was given through Moses; grace and truth came through Jesus Christ. (John 1:17)
Hebrews 12 also compares the two periods, confirming the same. The old covenant is characterize by “fire; to darkness, gloom and storm” in the new covenant you “have come to thousands upon thousands of angels in joyful assembly”.
Even the different miracles between the two illustrate the contrast in color. Under Moses there were many miracles bringing death, as a representation of the ‘curse’ under Law. In the new, Jesus healed every sinner who came to him with faith and need, representing the reversal of that curse.
This is all harmonious and faith building to him that ‘has an ear to hear’.