Here is another (3D!) model of thinking with assent as the act of faith:
According to this model, faith increases when both thinking and assent increase; it decreases when either thinking or assent decreases; and it remains the same when only one of the two increases.
In other words, if you do a lot more thinking than assenting, the level of your faith is indicated by the level of your assent; if you assent more than you think, your faith is proportional to your thinking.
Thus the whole dark blue region is "little faith," and faith increases up to the red region, which is "great faith."
"Lord, if it is you, command me to come to you on the water." The strangeness of this statement can pass us by in the whole drama of the story, but I'd say it was spoken in a moment when Peter was not engaged in top quality thinking. Still, he seems ready to do whatever Jesus might tell him, so I'd plot this at about an assent of 7 and a thinking of 3, in the lower right light blue.
Peter got out of the boat and began to walk on the water toward Jesus. If you're walking on the water toward Jesus, they can say what they like about you, but you're acting in great faith. This gets plotted up in the orange.
But when he saw how strong the wind was he became frightened; and, beginning to sink, he cried out, "Lord, save me!" Suddenly, Peter is doing way too much thinking and way too little assenting. Dark blue, upper left.
"O you of little faith, why did you doubt?"
There's something sort of Sisyphean about this plot, or at least about plotting Peter's actions on it. We want to keep pushing up toward the red, but if we push too hard in one direction, we might wind up rolling down the opposite slope. Of course, the solution is for us to only push when Christ is pulling.