I hear this phrase a lot, and interestingly enough, I hear it from liberals and conservatives, from Protestants, Catholics, Jews, Mormons, Atheists. It doesn't matter.
One variation of this phrase goes something like this: "We should not believe in a God who would be capable of doing _____." (Commanding Abraham to sacrifice Isaac; commanding war, conquest and genocide in the Old Testament; requiring that a Savior die on the cross to forgive our sins; commanding the practice of polygamy, etc. Fill in the blank.)
Everyone (including folks who don't actually believe in God at all) has some idea about what God is about, what God has done, does, and will do, what God is capable of and what God is incapable of. I have been/am/will be guilty of this as much as anybody else.
Like all opinions, some of these opinions are better informed than others. Some are based on a better awareness of scriptural testimony regarding what God has done in the past than others. Some are based on personal experience with a living God. But those who know God best (and I'm specifically thinking of numerous scriptural witnesses of God) insist that none of us, no matter how acquainted we may feel with the ways of God, knows God well enough to predict exactly what he will and will not do. Even those prophets with the most intimate personal knowledge of God have repeatedly been surprised, amazed, and humbled by revelations of God's will that they never expected.
Those who are wise content themselves to wait patiently for God to manifest his will rather than make predictions about it.
Losing faith in God because he doesn't manifest his will on our timetable or in the way we would like is literally one of the oldest forms of impatience in the book. Cain offers God a sacrifice thinking, God will like this! God doesn't like it. Cain was wrong about what would be acceptable to God. God offers Cain a second chance. He simply says to Cain: "If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him" (Genesis 4:7). In other words, if we let go of our expectations of what God asks of us, if we listen and follow, we will "do well." If not, we will fall under the "rule" of unrighteous desires.
It's such a simple, fundamental principle. And it all boils down to waiting and listening and letting go. It also boils down to trust.
A key moment in my own journey with God was when I let go of my assumptions and misunderstandings of God's dealings with others. Just because God has asked "X" of somebody else, and just because I imagine that to be unpleasant, or painful, or distasteful, or unfair, doesn't mean that I fully understand the context, and it doesn't mean that that is what God is asking of me. Maybe God will ask it of me, but maybe not. I need to be patient. What I know of God's dealings with me is that God has never let me down. Whenever I've trusted in what he's asked me to do, I've prospered and found joy and peace and done well -- even in difficult circumstances, even in illness or loss or poverty or times of loneliness. So if those have been God's dealings with me in the past, I can trust that whatever his dealings with me are in the future, they will work out for the best.
What I see of God's dealings with others is only half (maybe less) of the story. To understand those dealings, I have to understand them from the inside out, and I have to see them through from beginning to end.
In a sense, I don't even fully understand all of God's dealings with me, because I haven't seen the end yet. I'm still in the realm of mortality, not in eternity.
What I do know is along the lines of what is described in Alma 32. I can see the good that has come so far. I can exist in my understanding of God's dealings with me on a spectrum between total ignorance and perfect knowledge, and faith is the vehicle that allows me to move toward the knowledge end of the spectrum. Way points on that spectrum include perfect knowledge of limited principles, but the end point of perfect, ultimate knowledge exists beyond the span of our mortal existence ("for now we see through a glass darkly," 1 Corinthians 13:12).
If faith is the moving principle, patience, hope and love are principle methods for exercising faith.
We can choose (like that old American political party founded on hatred of Catholics) to "Know Nothing." We can choose not to know God. But the invitation to know God, to let go of our prejudices and our assumptions and to let God reveal himself to us, is always there, like it was to Cain.