No, it's not!! As Dr. King stated, the church is not the master NOR the slave of the state. It is its conscience.
Yet, we have holders of our highest office (current one included) take that oath before God. We have in our Declaration of Independence that the Creator gives us inalienable rights.
That is NOT the hallmark of a secular nation.
Our laws are based, in part, on the Ten Commandments (recently re-instated in a courtroom in Kentucky).
That is NOT the hallmark of a secular nation.
Here's the oath of the POTUS.
Can you show me where the God part is?
You've claimed it's there right?
"I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States."
Here's another clue for you that we are absolutely a secular nation:
Article 6 of the Constitution states:
"No religious test shall ever be required as a qualification to any office or public trust under the United States."