There are a lot of things Christians “need to know.” For some it’s predestination, for others, the age of the earth, or the order of end times events. In reality, the core of theology is simpler than that.Mike Bird in his, Evangelical Theology reminds us of the test for Christian theology:

The Sermon on the Mount (Matthew 5– 7) and the Sermon on the Plain (Luke 6) are good test cases for any theological system.

Contra some Reformed theologians, Jesus is not teaching people the law so they can see how they don’t measure up, wail for their sinful hearts, and realize their need for the imputation of Jesus’ righteousness. Contra some dispensational theologians, Jesus is not teaching what kind of law the Jews will keep in a post-rapture millennium. The Sermon on the Mount is Jesus’ manifesto for the kingdom. It is the ethical vision for God’s people if they are to live out the covenantal righteousness that comes from experiencing the kingdom’s saving power. This is what the new Israel of the new age is supposed to look like. Not the elitist micropiety of Pharisaic leaders who claim their tradition represents the true measure of righteousness, nor the compromised Jewishness of the Herodians who dress up Hellenistic values in a Jewish garb. The sermon is about new law for the new age.

Bird, Michael F.; Bird, Michael F.. Evangelical Theology: A Biblical and Systematic Introduction (Kindle Locations 8394-8401). Zondervan. Kindle Edition.

Christian theology must accommodate the teachings of Jesus rather than circumvent them. If it cannot, it is not Christian.