What is the reason Christians had such a widespread appeal?

Because the religion teaches to protect the weak, respect the parents and be nice to strangers. You never know, one day you may be reaching the stars, but couple of days later you may be lying down with your face in the gutter.

The three partite "the father, the almighty father, and the holy spirit" is actually very symmetric. If you are a woman you will have three most important men in your life: your husband, your son and your father.