How do we Reconcile the God of the Old Testament with the God of the New Testament

Same God, different expression. 

The God in the New Testament is not different from God in the Old Testament. 

God never became more or less to be God, God was God all along.

God only became a man in the person of Jesus to teach about himself. And still he was God. (Hebrews 1:3, John 1:1-14)

God in the beginning is God today, tomorrow and God forever. (Hebrews 13:8, Malachi 3:6)

Reconciling the God of the Old Testament with the God of the New Testament

In the Old Testament, God reveals Himself to man specifically the children of Isreal in types and shadows. 

God did this, because men needed to grow in the understanding of God, learning God as children. Who today, are being told don't do this, don't do that, do that. All of this law is not to hurt them, but to protect them from hurting themselves. So God in the beginning was instructing and reordering man to live right with love. First teaching us how to love one another before we can then say we love Him.

In the same old Testament, God was being called in names, which I will be listing a few. (Elohim -Genesis 1:1, Yahweh - Exodus 6:3, Adonai - Psalm 8-1)

All the names was birth out of men's understanding of the same one true God.

And in the same Old Testament, the Holy Bible should how men was growing in faith to understand God. Like Abraham, Moses, David and Many others grew in faith to understand God. (Genesis 15:1-6, Exodus 3:1-15, Psalm 23)

 And in the New Testament, these men where referred to as men of faith. (Hebrews 11:1-40, Romans 4:1-25)

All of the differences in expression was tied to an understanding of who God is.

Some call him;

Judge, lawgiver, God of Justice, God of Mercy, Our Creator, Our Helper... and then the Son Jesus came and thought us the best name to call him father.

He called the children of Israelite children, still they never called him father, instead they kept seeing him farther, even when he was with them in everything. 

They cherished everything he gives, yet never cherished his person, until his son now came to draw us closer. 

Now we call him father with a better sight and understanding through his son, whom John in the gospel refers to as The Word of God (John 1:1-14)

All of these, was man coming to an understanding of the one true God

Something to note in reconciling the God of the Old Testament with the God of the New Testament.

In the Old Testament, God spoke to men, through men, using kings, prophets, and writers of old and theses where all written as ( The Law, the Prophets, and the writtings)

In the New Testament, God gave his word through his son whom everything was writing in pointer to. And these were all writing through the Apostles of Jesus Christ called the Gospel, and in conclusion gave a summary of it all. Telling us in his letters to the churches the essence of the New Testament which emphasis Christ teaching through the gospel. (John 1:1-14, 1Corinthians 15:1-58)

And it's to the end that God is seen, heard, read, revealed and glorified.

 In conclusion

The Old Testament was not Old, because it's old, but because they was more God was to reveal to man which was then revealed in the New.

God gave the law through Moses, and Grace and truth He gave through his son and this God give in abundance. (John 1:16, 17)

The Law was not what God intended to give men from the beginning, but to teach me from the beginning everything about himself. Wrapped in Faith and Love, given as Grace.

He first gave the promise, the law, the prophet through whom the gospel was foretold and the person of whom it was written about. The same came to teach us the one through God we all know and believe today.

Everything written was God establishing his word and it was all written for our learning. (2Timothy 3:16-17, Romans 15:4, 1Corinthians 10:6)

Thank God for Love

Thank God for the word

Thank God for He came

Thank God for the work

Thank God for the old

Thank God for the new 

Thank God for his Son

Thank God for his Holy Spirit 

Thank God for reconciliation

Thank God he made and call us children

Thank God we are all one in Christ Jesus. 

Amen 🙏 🙏 🙏 

Have you noticed the Holy Bible is always referred to as THE WORD OF GOD, not the words of God.

That's the summary of it all.

Everything was interwoven into God's perfect plan including you and I who God had something better in mind for, so that they would not reach perfection without us. (Hebrews 11:39 NLT)

We are here to perfect the work God started in his word through his son.

For further references, Read the whole of Chapter of Acts of the Apostles, The Gospel of Matthew, Mark, Luke and John, the Letters of Apostles to the churches, Romans, Corinthians, Galatians, Hebrews, etc.

All that was happening in the New Testament was reconciliation.