Great question. I had similar feelings of two different Gods, until I studied and the answer came to me. OT period was the period where He established what his kingdom is and what is required. The laws given, the cleanliness required, the strict temple rules, all are there to mirror His requirements being a just God. Even the miracles He performed serves a purpose of foreshadowing. When Jesus was born, there's a reason the jews of that time expected a grand king to charge in and destroy their enemies. Up to that point that was what God has established for his people, a seemingly angry and jealous God, but one who rewarded obedience from His people. But instead, He sent a small, humble baby born in a literal animal stable and son of a carpenter parent and no account mother. Not exactly God material, you'd think. It's also why during His ministry He had such a hard time convincing the jews of the region of His divinity. Who is this simple man before me? They SAY he performs miracles but who says it's just magic and showy tricks? And yet, when you read the OT, everything point to Jesus being the Christ when you have the benefit of hindsight. The Father has the strictest demands of purity, being a holy God. The son was sent to reconcile this, since humans can NEVER be holy. We can never live to the law. We can never achieve, much less maintain, the standards required to stand in his presence. So God sent his Son to be the perfect sacrifice for us, as for told and required by prophecy, to tear down the veil between us and the Father. We love live in the post sacrifice era. Faith and trust replaces the requirements of an impossible law. We can approach God directly on prayer and worship rather than needing the temple and priests for our behalf. We have the perfect sacrifice rather than needing to sacrifice animals for us to temporarily attone for our sins. And most importantly: God no longer chases after a single nation of "jews", holding them to strict laws and retribution (although he still punished even in our time), but now extends himself to all peoples and all lands, and anyone who accepts Jesus will be considered His people. Similar to the snake on the pole in the time of Moses, we need only humble ourselves and look up to him, and be saved.