The Ten Commandments given to Moses on Mt. Sinai were the terms of a Covenant with the nation of Israel.
We know that the History of Israel records their continual disobedience to God and to His covenant. Because they broke their covenant with God, they were scattered over and over again, until finally they nation of Israel was judged in AD 70 during the destruction of Jerusalem by the Romans, as Jesus predicted it would be (Luke 21) and in the Parable of the Vineyard (Matt. 21:33-46).
“As you come to him, the living Stone—rejected by humans but chosen by God and precious to him— you also, like living stones, are being built into a spiritual house to be a holy priesthood, offering spiritual sacrifices acceptable to God through Jesus Christ.” (1 Peter 2:4-5)
“But you are a chosen people, a royal priesthood, a holy nation, God’s special possession, that you may declare the praises of him who called you out of darkness into his wonderful light. Once you were not a people, but now you are the people of God; once you had not received mercy, but now you have received mercy.” (1 Peter 2:9-10)
Here, Peter declares that Christians “ARE a chosen people, a royal priesthood, a holy nation, God’s special possession…”. The very same conditional promises originally offered to the Jewish nation in Exodus 19:5-6 are now spoken to the Church as being now in effect. So that anyone who is currently found in Christ is the recipient of these promises to be chosen, of the priesthood, a holy nation and God's special possession. We are also promised to be called the people of God and to receive mercy.
The Good News is that Jesus first came and fulfilled the terms of the Old Covenant, and then He made a New Covenant with anyone who would receive Him as Lord and Savior. The terms of that New Covenant are simple, “I will put my law in their minds and write it on their hearts. I will be their God and they will be my people…and they will all know me from the least to the greatest for I will remember their sins no more.” (Jer. 31:33-34 and Hebrews 8:7-9)
We are now under one Covenant, not two. The first has been fulfilled and is now obsolete:
“In speaking of a new covenant, he makes the first one obsolete. And what is becoming obsolete and growing old is ready to vanish away.” (Hebrew 8:13)