Astrology denotes character and is a marker of events, or destiny. It doesn't tell us how to respond to a particular event or what the outcome will be. That's up to us and, therefore, we also have free will. Destiny and free will exist simultaneously. They are yin and yang.
I believe that we each have a destiny, we each have events that we will have to face in life, and we can easily see those in a transit chart. Our character (composed of our natural leanings and our growth/decline through circumstances) determines how we will respond and, therefore, determines the eventual outcome. We each have the capacity to improve our character, thus changing the outcome of our destinies.
A Saturn transit through the 7th house will imply difficulties with, and perhaps the end of, a major relationship. That's destiny. We can site example after example of this happening in gazillions of charts.
How I respond to those difficulties and the choices I make will determine the outcome. That's free will.
This is why directed prayer, meditation, analysis, or looking inward are all fairly important to the best outcome of a destined event. It can help us to make wise choices.
With the Saturn/7th transit example, I can decide that my partner is a jerk and I'm not going to put up with it anymore. And maybe that's the best choice sometimes. Or I can do some work the faults I brought to the relationship, pay close attention to the nurturance of the relationship and my partner, thereby saving my relationship and changing my destiny.
To the "prayer changes everything" argument--that's just not true, although it might be nice if it was. I may have a major test coming up, and I must score a 94 to pass. I haven't been a very good student, so I decide to pray instead of study. What's the chance I'll pass that test with a 94?
I can study diligently, without prayer, and have a higher likelihood of passing the test.
If I study diligently, and offer a prayer of faith (which may boost my confidence and give me some ease), I will probably (hopefully) do quite well on the test.
The test is destiny. How I respond is free will. Study and prayer are both exceptional tools.