•Jesus is Lord
•The gospel message/apostolic kerygma that was proclaimed
•Historically speaking, the origin of Christianity was founded on the essential, non-negotiable 'good news' proclamation of Christ's atoning death & bodily resurrection, which to the earliest Christians evidenced Christ's vindication and exaltation as Lord by God the Father
E.g., Pre-Pauline Creed of 1 Corinthians 15:3-5 ("For I delivered to you of first importance...)
•Importantly, the most essential Christian tenet is not so much a tenet or doctrinal statement, as it is an event (i.e., the bodily resurrection). Theologically, everything else (e.g., the Torah, salvation, kingdom of God, eschatology, final judgment, corporate membership in the Body of Christ, ethics/morality, Jewish monotheism/the Shema expansion to include Christ alongside Yawheh, while denying all other gods, etc., etc.) is reframed in light of this one, single history-altering event of the resurrection.
*In different periods of history, the church has overemphasized certain theological aspects at the expense of others as well as added doctrines that have detracted from the centrality of the gospel message/apostolic kerygma. Any discussion of essentials, would necessitate having to weed through those traditions and imbalances that have developed in Catholicism, Protestantism, and East Orthodox traditions.
*The classic academic work on the subject that is credited with first bringing scholarly attention to the historical centrality of the crucifixion-resurrection event in origins of Christianity studies was:
CH Dodd's The Apostolic Preaching And Its Developments