This is a good point for Steven to consider -- you're using a phrase that some people consider part of their sacred life, so it could potentially offend someone if they think you're being flippant about their faith just to get a "cool" tattoo.
If you're not totally sincere about your own belief in God, you could always omit that word and just get a tattoo that says Прости меня, ибо я согрешил -- without specifying whom you're asking forgiveness from.
Personally, I would go with:
Помилуй меня, Бегемот, ибо я согрешил!
In a circle around an image of this guy:
...but that's just my own religious preference, and anyway I can't get tattoos because my skin has a tendency to form keloid scars.