The good news for those of us who struggle with the call of God in our lives is that although it seems to require either some amazing feat of imagination or some incredible strength of will, responding in faith to God's leading requires truly nothing more than faith and love.
It doesn't matter if I can imagine what God says He's up to. It doesn't matter if I can see it. It would be nice, of course, and there is a certain part of me that is keenly aware of how often God says, "They have eyes, but they cannot see." And then I find myself crying out, "Lord, I have eyes! Let me see!" But the truth is that I don't really need to see. I don't really need to be able to fathom.
Because I already know.
I know the character of the God who calls me. I know His faithfulness. I know, without a doubt, that I can trust Him. If I can only bring to mind all of the things in which I am sure about my God, then my limited imagination does not, and cannot, stand in the way of my following Him.
And if I remember all of the reasons I have for following Him, all of the assurance I have in trusting Him (faith), then my reluctant will is also not a problem. However much I may not want to do the thing that God has called me to do, I profoundly want (read: will) to obey God. The passion of the will of faith supersedes the hesitations of my will to my own desire.
So the response of the faithful must be an absolute confident assurance in the character, nature, and promise of God. Then, all these little human things that we fight so strongly against just don't matter any more. They don't stand in the way of our doing a faithful thing.
Abraham heard God's call to be a father of many nations. He laughed, yes, but then he had a son anyway. (And they named him, "he laughs.") So when God then calls Abraham to sacrifice that son, to climb Mount Moriah, to continue journeying into other lands - all things which may be against Abraham's will - then all Abraham has to do is trust in the character, the nature, and the promise of this God who has already proved faithful. He doesn't have to love where he's going, but if he loves God, he'll go. And who else has made such a promise to Abraham, let alone delivered on it?
Moses heard God's call to be a leader of the people. He couldn't see it. But he'd already spent enough years in the wilderness, his heart aching for his people, worshiping the God he'd kept in secret as a kid in a king's palace. He had a foundational understanding of this God who called him, and all of those holy encounters they'd already had were enough to convince Moses that this God was the real deal. And if God is the real deal, then there must be some real reason He's asking you to do what He's asking you to do. So Moses leads the people, not because he's sure of himself, but because he's sure of his God.
Noah was a righteous man; this is well-documented. A righteous man has a deep and rich relationship with a righteous God. He has to. And the very fact that this God has noticed Noah's righteousness and chosen him as a potential remnant of the entire race of humanity only further confirms Noah's sense of God - He does notice, He does care, all this hard righteousness that Noah has invested his life in in a tough place does mean something. And if it all really means something, if it does truly matter, then there's something legitimate about this God. There's something worthy of Him. If God really is paying enough attention to know what one man does, then this one man can trust the investment that this God is making in him. Therefore, if this God wants a boat, then let's build a boat. If this God has made the investment in men, then let men make the investment in this God, knowing that the return is right there.
Jonah was a prophet. He had spoken for God on numerous occasions. He had seen first-hand how God's truth rains down on His people. He knew the way that the word of God came to him, and he knew how that very same word was fulfilled. We don't know any of the other scenes of Jonah's story, but we know this is generally true of the prophets. (And fun fact: the book of Jonah begins with a Hebrew word that means "and," which means that we are told right away that we're not getting a comprehensive story of Jonah, but only one scene of him.) I think God had a special obligation to the prophets to show His faithfulness. After all, if you're going to ask someone to speak powerful words for you, they have to know that you're going to stand behind them. Don't they? So we can only assume that Nineveh was not Jonah's first rodeo. And even if it was, that lingering stench of fish vomit ought to be enough to remind him of God's faithfulness. So he may not want to go to Nineveh, but if he knows that God is powerful, and if he trusts in this, then he'll go. He'll go because he knows that God is going, too.
At any given moment, we are not required to believe in ourselves. We are not required to believe in our journeys. We are not required to believe in our stories. All we are ever required to believe in is our God. If we know who He is, if we trust in His character, if we know His nature, then everything else is naught. Our limited imaginations and our hesitant wills will not, and cannot, hold us back.
So the question that may be most pressing in our lives may not be what God calls us to be or what God calls us to do, but who this God is who calls us at all. If we can answer that with any reasonable confidence, then we'll go. We'll go because we know the One who asks us to go.
And that's enough.