Mr Lee is going to challenge the idea that a business can have religious beliefs and argue that owners' opinions are separate from their company's.
In 2014, Mr Lee asked a Christian-owned bakery business to make him a cake that said 'Support Gay Marriage' on it, which they refused to do due to their Christian beliefs.
After losing in a lower court, the McArthur family who own Ashers bakery won their case at the UK Supreme Court in 2018, with Justice Lady Hale saying they had not been discriminatory for refusing to bake the cake.
She said that was because they weren't refusing the customer because of his sexuality but were opposed to the message - no matter who had asked for it.
Now, Gareth Lee, a gay rights activist, is taking the case to the European Court of Human Rights, arguing that the UK court "failed to give appropriate weight" to his human rights.
Justice Lady Hale addressed the couple's freedoms as stated in the European Convention of Human Rights in October 2018, saying: "The freedom to not express an opinion which one does not hold is also protected by Article 10 of the convention...
"This court has held that 'nobody should be forced to have or express a political opinion in which he does not believe".
Daniel and Amy McArthur lost their initial case before they won at the Supreme Court.
The Equality Commission first took up Mr Lee's case in 2014 and it went to court in Belfast with them claiming the McArthurs had acted with discrimination.
Now, the case will be taken up by Phoenix law, who said in a statement: "There is no such thing as a 'Christian business' and we contest that no such thing ought to be given legal recognition by a court."
David Smyth, public policy officer for the Evangelical Alliance in Northern Ireland told Premier: "The issues remain around religious belief, political opinion and the right not to be compelled to produce goods with a certain message. So, a lot of the same issues will remain
"It will be interesting to see where this goes from here on what ultimate outcome will take place and whether that will change the way Ashers bakery or other businesses who are owned by Christians are able to run their their businesses and supply goods and services."
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