Because of the word of God and the testimony of Jesus..Rev1:9
With Jesus, they made Him confess who He is, they asked Him if He is the Christ. He gave the wrong answer (in their minds) and this enraged the Sanhedrin, they killed Him.
In the early church era, people had to confess “Caesar is lord”, (if they confessed Christ is Lord they were killed) in the middle ages, people had to
bow to the pope and confess that the pope is the “stand in” of Christ on earth, (if they confessed Christ alone they were killed)… in the end people will have to confess that fake Israel is God’s true Israel or is “the savior”……..if not you must be killed. (Figuratively and/ or literally.)
All 3 of these scenarios involve the flesh, “divine right of kings” that held up in the kingdoms of the earth……in that Caesar, the pope (and the earth’s crowned Kings&Queens) and false Israel were/ are considered DIVINE rulers of the earth (royalty, kings) and either representing God or God Himself.
Oaths or not taking oaths, and confessing or not confessing, are a big part of the drama that plays out in the tribulation.
Confessing Christ Jesus as the only Savior, the only Christ (the only true GOD) worthy of obedience, and the only King worthy of allegiance, helps separate everyone in the (ongoing) tribulation of the saints. Regardless of the consequences (Rev.12:11), the overcomers (Rev.3:21) are loyal to the Kingdom of Christ, pledge allegiance to Christ alone, and only confess the Truth.