The Jews are the descendants of a Hebrew named Abram.
The Old Testament is the record of God’s dealing with men from the creation of the world.
It explains that God’s purpose is to fill the earth with his glory in the form of men and women raised to the divine nature in whom and with whom God will live forever.
It describes creation and the disobedience of Adam and Eve which brought sin and death into the world.
Sin is disobedience to God’s precepts.
The penalty for unforgiven sin is eternal death.
It describes the confusion of language and the spread of the families to become nations.
It describes God’s choosing of a man called Abram whom he renamed Abraham.
It describes the promises God made to Abraham of eternal inheritance on the earth to be made possible by one of his descendants.
It describes God’s dealing with Abraham’s descendants who became a nation called Israel.
It describes how Israel became a kingdom and God’s choice of David to be their king.
It describes God’s promises to David that one of his descendants would be established upon his throne forever.
In this descendant of Abraham and David all families of the earth will be blessed.
This then is the Jewish religion – They are awaiting a descendant of David called the anointed one – Messiah (in Hebrew) or Christ (in Greek).
The New Testament describes the birth and ministry of the promised Messiah, called Jesus.
Jesus is the son of God and the promised descendant of Abraham and David through Mary, his mother.
The blessing is made available to all families of the earth through the gospel of the kingdom of God preached by Jesus and his apostles.
Jesus lived a sinless life and voluntarily gave it in sacrifice for the forgiveness of sin.
God accepted his sacrifice and raised him from the dead.
To have our sins forgiven we must believe the gospel, be baptised by immersion in water for the remission of past sins and live a new life by Jesus’ commandments which are found in the New Testament.
Many of the Jews believed in Jesus but the majority didn’t.
They caused him to be put to death by the Romans.
This was clearly prophesied in the Old Testament, but they wouldn’t (and still won’t) believe it.
After Jesus’ resurrection the gospel was opened up to the gentiles (non-Jews).
Those who believed and obeyed the gospel (both Jews and gentiles) became known as Christians.
The Jews accept the Old Testament and are still waiting for their Messiah. Christians accept the Old and New Testaments and believe that Jesus is the Messiah and are awaiting his return from Heaven to establish the kingdom of God.
When Jesus returns the unbelieving Jews will recognise their mistake and be reconciled to God.
Jesus will reign over the mortal population of the earth for 1000 years before the final judgment.
Following this God will be all in all.
Paul explains what is to happen:
1 Corinthians 15:20 But now is Christ risen from the dead, and become the firstfruits of them that slept. 21 For since by man came death, by man came also the resurrection of the dead. 22 For as in Adam all die, even so in Christ shall all be made alive. 23 But every man in his own order: Christ the firstfruits; afterward they that are Christ’s at his coming. 24 Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power. 25 For he must reign (the 1000-year reign), till he hath put all enemies under his feet. 26 The last enemy that shall be destroyed is death. 27 For he hath put all things under his feet. But when he saith all things are put under him, it is manifest that he is excepted, which did put all things under him. 28 And when all things shall be subdued unto him, then shall the Son also himself be subject unto him that put all things under him, that God may be all in all.
Sadly, the Christian Church fell into apostasy in the 4th century AD and incorporated many false doctrines – trinity, immortal souls, heaven-going, eternal torment in hell, Satan as a fallen angel, etc.
The availability of printed Bibles in common languages brought about what is generally called the Reformation. Following this, many churches sprang up with different gospels and doctrines.
They can’t all be correct.
The scriptures which make up the Old and New Testaments were all given by inspiration of God and contain the truth to be believed and received.
Church gospels and doctrines must be compared to the scriptures to determine the truth.
The promise in the gospel is that faithful believers will be raised from the dead to eternal life in the kingdom of God upon earth when Christ returns.
May I suggest that you read:
Your Share in God's Promises: https://www.thisisyourbible.com/library/show/181
I hope you find this helpful.