Not buying it.
Christians the world over watching the Supreme Court decision on religious freedoms for the Christian baker who was sued by the lesbian couple for not baking a cake for their wedding. The Supreme Court overturned both lower courts' decisions and upheld the baker's religious right to refuse to bake the cake.
At this point, no Christian is going to sue you for your lack of desire to paint a religious-themed painting of them because they already know religious freedoms are upheld by this Court; ergo, if they did file a suit against you, it would be tossed.
However, I find it interesting that people were all over the Christian bakers refusing to bake the cake, all gung-ho for the lawsuit, all gung-ho to put the bakers out of business citing "hate." And those same people will applaud you for refusing to paint a religious-themed painting for a Christian family, also based on hate.
However, as a Christian woman, I agree that you are well within your rights to refuse to paint any portrait of anyone, for any reason, at any time, even citing your own religious beliefs. If Constitutional rights fall for one, they fall for all.