Judaism is Biblical. The holy days are from the Bible. The Law is God’s Law. Jesus or Yeshua (Yeshua is Jesus’ Hebrew name) is the Messiah whether a person is a Jew, Jewish (or practices Judaism and/or the things of the Law), or Christian. True Judaism accepts the Messiah and the new covenant so nothing is left out.
As a Christian I already accepted the Bible. But I discovered that the new covenant is not a new religion (Christianity was never a new religion, Christian practice originated from or in Judaism). The early church was entirely Jewish. That is, the Law is God’s Law and we ought to obey God whether we understand everything about it (God’s Law) or not. If you seek to understand God’s Law you will find you can move forward with your life and understanding of Biblical and spiritual things rather than just seeking to understand things that some might view as old and outdated. The Law is actually a foundation for understanding who Jesus is and what He has done for us. It helps us to understand who we are as individuals.
I have questions about the new covenant, because I know it is not less it is more. But I also understand it is different. That is, it is not the same covenant. So I can say God’s Law is never not God’s Law. And I can appreciate the new covenant now in light of the old. Whether Jew or Gentile there is one God. The Messiah (Yeshua, Jesus) brought us the new covenant. We have forgiveness of sins which though it was not absent under the old covenant carries with it the promise of being able to live a righteous life in Yeshua. And, we have eternal or everlasting life. These things are because of the power of God in saving people from their sin and purchasing them back for Himself in the death of Yeshua the Messiah who innocent shed His blood for us. The resurrection of Yeshua shows God’s power for us, power over death, when death was the consequence for sin, for those of us who are of faith (believing God and the promises of God, believing God at His word).
Christians don’t always understand the righteousness, holiness, and justice of God. But as Christians we can come to understand these whether we chose to identify as Jewish or not.