Yes, he can.

Article II of the US Constitution:

Before he enter on the execution of his office, he shall take the following oath or affirmation:--"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."

The First Amendment guarantees freedoms concerning religion, expression, assembly, and the right to petition. It forbids Congress from both promoting one religion over others and also restricting an individual’s religious practices. It guarantees freedom of expression by prohibiting Congress from restricting the press or the rights of individuals to speak freely. It also guarantees the right of citizens to assemble peaceably and to petition their government.

His duty is to the Constitution and specifically, in this case, the First Amendment to the Constitution.