But Abram said, "Lord God, what will You give me, seeing I go childless, and the heir of my house is Eliezer of Damascus?" Then Abram said, "Look, You have given me no offspring; indeed one born in my house is my heir!" And behold, the word of the Lord came to him, saying, "This one shall not be your heir, but one who will come from your own body shall be your heir."
Henry Drummond, a 19th-century Scottish evangelist, observed, "Christ never failed to distinguish between doubt and unbelief. Doubt is can't believe; unbelief is won't believe. Doubt is honesty; unbelief is obstinacy. Doubt is looking for light; unbelief is being content with darkness."
When God promised to bless him (Gen. 15:1), Abraham responded, "How are you going to do it?" This was not a lack of faith, because Abraham believed God was going to keep His promise; he simply didn't understand how God would do it. Since "blessings" were always believed to come through children (Ps. 127:3-5), and Abraham had none (nor from a human perspective did it look like he would have any), he inquired as to how God was going to do it. God honored that question and told him, "One who will come from your own body shall be your heir." Abraham was satisfied. The case was closed.
It's not wrong to ask questions about God's plan; it's only wrong to question the rightness of God's plan. There's a subtle but important difference here. It's only natural for people to want to know, "God, how do You plan to pull this off?" In essence Abraham said, "I'm to be the father of a great nation, and yet I have no children. I know You can do it, Lord. But I'd sure like to know how."
If you're wondering how God will work His will and His way in your life, it's perfectly legitimate to ask. But always ask in faith. Feel free to question how God is going to work out His plan, but never question His ability to work out that plan. The first is inquiring faith; the second is irreverent unbelief.
Doubt asks how; unbelief asks why.