A five-year legal battle is set for it's conclusion as Phillips battles for his right to refuse to make a gay wedding cake.
In 2012 a same sex couple entered his store, Masterpiece Cakeshop, in Colorado to ask him to design a cake for their wedding.
When he refused, the state intervened.
Supported by religious freedom charity Alliance Defending Freedom (ADF), he will have his say at the USA's highest court, with the verdict set to create a precedent across the country.
On Tuesday morning, hundreds gathered outside the court to show support for Phillips.
His main legal argument is that no one should be forced to create artistic expression that communicates a message with which he fundamentally disagrees.
Speaking about the impact the case could have on Christians across the USA, Jeremey Tedesco from ADF said: "If the government has the power to coerce Jack Phillips to create artistic expression that violate his beliefs then it ultimately has that power over all of us.
"The principle established in this case is going to be very important moving forward."
The case mirrors the legal action faced by Ashers Bakery in Norther Ireland.
The owners were found to have discriminated against a customer based on his sexual orientation after they refused to make a cake which said 'support gay marriage'.
Having been told to pay compensation, they're still trying to overturn the decision.