What is the Main Character choosing between when he/she makes a Leap of Faith?
The answer is that the Main Character needs to make a choice between going with the MC Solution, or going with the MC Response when he makes a "leap of faith" at the personal crisis point in the Main Character throughline.
FOR CHANGE CHARACTERS
Change characters come to a story with baggage. The baggage consists of the MC Problem hidden by blinders (layers of justifications), which are replaced by the MC Symptom as the apparent source of the MC's personal troubles. In comes the Impact (Influence) Character who disrupts the MC's equilibrium by introducing an alternate way of addressing the MC Problem -- one that MAY be more effective.
As the acts progress, the MC's blinders are torn (or worn) away. By the end of the story, all the blinders are removed and the MC can now see BOTH pathways open to him. The argument for continuing to use the MC Response to address the MC Symptom is balanced by the potential effectiveness of the MC Solution to resolve the MC Problem. Unfortunately for the MC, the amount of pressure applied on the MC by BOTH approaches is such that the MC must choose one or the other or risk a psychic break. Since there is no telling which approach will resolve the personal inequity of the MC, the MC must make a 'leap of faith' by choosing to discard his old way of doing things and adopt the MC Solution with the hope that he makes the correct choice (for him).
FOR STEADFAST CHARACTERS
Steadfast characters' stories are slightly different. Generally-speaking, steadfast MC's do not have baggage at the beginning of the story -- at least baggage related to the personal issues to be explored in the story. Then, SOMETHING HAPPENS to throw the MC off balance (MC Symptom), which requires the MC to make a choice or do something in response (MC Response). This response to the event becomes the cornerstone of his apparent inequity. (For example, Dr. Richard Kimble's wife is murdered and he is wrongly convicted for her death.)
Unlike a change main character who comes fully-loaded with back story baggage and whose resolve is eroded as the story progresses, the story of a steadfast main character is the process of building UP the blinders. This works to focus the MC's resolve on the MC Symptom over the MC Problem (which acts as the source of the MC's drive). The Impact (Influence) Character performs the same function by increasing the pressure on the MC. With each act turn, the MC must shore up his resolve to counter the pressure placed on him by the IC.
By the end of the story, the amount of effort it takes the MC to maintain his resolve, matched by the IC's pressure to force him to change, threatens to overload the MC completely. Like a Change Main Character, the Steadfast Character cannot tell if continuing to try to resolve his personal issues by pursuing the MC Response is better than throwing away the source of his drive (MC Problem) and adopting the MC Solution. So, he makes a 'leap of faith' by blindly picking to stay the course and hopes it will resolve his personal issues.