I am no expert; I only read a lot. But, my understanding is that early Christianity was a reaction away from Judaism. And, that the Old Testament was only added as part of the canon later. In contrast to Judaism, Christianity was must more similar to Buddhism with it’s hippy love message. But, yes, I’m sure that’s very controversial.
Early Christianity is the fulfillment of 1st Century Judaism with Jesus Christ being revealed as the Messiah and King. Modern day Judaism, that is Rabbinic Judaism (which followed some time after the destruction of the 2nd Temple) denies Jesus as that expected Messiah and has an expectation of someone else that is to come.
For Christians, The Old Testament and New Testament are very much coupled together. There's a lot of prophecies and typologies you can read about from the Old Testament that finds its fulfillment in Jesus and the New Testament (such as the suffering Messiah).
I'd suggest C.S. Lewis' book "Mere Christianity" (who was good friends and influenced to convert by J.R.R. Tolkien and G.K. Chesterton) to get a clearer picture of Christianity without the sugar coating we commonly get from the media today. Lewis brings up the Trilemma, which is unique only to the founder of Christianity.
It’s also worth explicitly noting that Christianity-as-a-Jewish movement and Christianity-as-non-Judaism took a couple centuries to shake out. I would also guess that, and there is decent evidence for, christianity at the time would have looked much more similar to a communist movement (natural communism, think potlucks, not marxism) than the religion we know today—it appears you can thank Paul, in part, for that.
Also, if you’re looking to desugar Christianity with empathy, I recommend Kierkegaard’s excellent Fear and Trembling. Lewis’s trilemma falls uncomfortably close to Christianity-through-proof genre of literature—I realize that’s not his intent, but it leaves a bad taste in my mouth. I’ve always found his The Weight of Glory to be more compelling.
