Is the God of the OT the same as the God of the NT?

Yes, they referred to the same entity, the One True Most High God.

Why did the God of the Old Testament act so differently from Jesus?

No one can fully understand God. He revealed Himself to mankind progressively.

Hebrews‬ ‭8‬:‭

7‬ For if that first covenant had been faultless, there would have been no occasion to look for a second.

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If God is perfect, why must He establish a second covenant? Did He err in the first covenant, prompting Him to create another one?

No, this relates to God's modus operandi of gradual revelation, also known as progressive revelation. It is the theological concept that God's plans were not disclosed all at once, but revealed gradually over time throughout biblical history. This concept helps to explain the developmental nature of certain theological ideas.

Examples of gradual revelation in the Bible include:

• The concept of God evolved from early Israelite monolatry to explicit monotheism.
• Messianic prophecies became increasingly specific from Genesis to the later prophets.
• The concept of the afterlife developed from vague references in the early Old Testament to clearer teachings in later prophets and the New Testament.
• God's gradual revelation to Abram
• God's gradual revelation to Joseph
• Why did God allow slavery?

This concept recognized that God communicated in ways appropriate to the recipients' cultural and historical contexts. God's revelation unfolded over time, becoming clearer and more complete as history progressed. The revelation culminated in the person and work of Jesus Christ.

Hebrews 1:

1 Long ago, at many times and in many ways, God spoke to our fathers by the prophets, 2 but in these last days he has spoken to us by his Son, whom he appointed the heir of all things, through whom also he created the world.

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The Bible tells a cohesive story of redemption, but it acknowledges the developmental nature of God’s revelation. Earlier parts of Scripture lay the groundwork for later, fuller understandings.

Hebrews 13:

8 Jesus Christ is the same yesterday and today and forever.

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The God of the OT and NT is the same God—one who is holy, loving, just, and merciful. The differences in how God is revealed in the two testaments reflect the unfolding of his redemptive plan through progressive revelation. The OT lays the foundation, and the NT brings it to fulfillment in Jesus Christ.