Stephen Miller's first day as the new White House Chief of Staff may not be until Monday ... but already he's making oaths and pledges to the president.

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Speaking at Mar-a-Lago yesterday ... Stephen said ... "It is the honor of my life to be President Trump’s chief of staff. And I swear to you, and I pledge to you, Mr. President, my absolute loyalty, my unwavering commitment, and my total devotion to your success and the success of this country."

Stephen -- who will replace the outgoing and ousted Susan Wiles -- had nothing but gushing words to say about his boss.

He said ... "Let me tell you, if it wasn’t for President Trump, the United States of America would not exist right now. This man saved America."

The former aide added ... "This man took a bullet for us. He faced down life in prison for us. He saved the Republican Party from the ash heap of history and has made us stronger, more popular, and more resilient than ever before."

Despite Stephen saying he saved the GOP -- the party suffered a huge midterm defeat on Election Night, losing control of both the House and the Senate.

We reported earlier ... Trump's personnel purges might not be over yet.

Trump said in a recent preview clip for his sit-down with "60 Minutes" that he hopes to be doing a "few changes" in his administration -- specifically within his cabinet.