"I knew in my heart that I could not put that message on the cake," Karen McArthur said.
She has been giving evidence at the second day of the case at Belfast's County Court.
The Equality Commission launched action against Ashers last May after it refused to bake a cake with an image of Sesame Street puppets Bert and Ernie below the motto Support Gay Marriage.
The baker's owners, the McArthur family, have always maintained their Christian beliefs prevented them from baking a cake which they say is against the teaching of the Bible.
"The problem was with the message on the cake because, as a Christian, I do not support gay marriage," Mrs McArthur told the court.
The director told the court she had been a born-again Christian since the age of seven and would always try to please God.
She said she initially took the order from the customer because she "did not want to embarrass him or have a confrontation in the bakery".
Ashers is accused of discriminating on the basis of sexual orientation but the bakery claims that's not true because staff were unaware of the customer's background.
A verdict is expected in five weeks.