Let's continue our imaginary conversation with T, shall we? Say he's old enough to be relatively articulate, and he comes to us, his parents, and says, "Mom and dad, I know I'm supposed to trust God and not be afraid, but I can't help it: I'm still afraid."
Whoa. Welcome to the fundamental problem of the Christian life, mi hijo. I hope this question doesn't come too late at night; but, of course, it will, and I will try to be profound and clear while barely keeping my eyes open.
I remember being in seminary the first time I discovered Mark 9:24b: "I believe; help my unbelief!" Where was this verse for the rest of my life, and why wasn't I aware that it was in the Bible? (Please, don't answer that.)
So what do I tell a little boy who has discovered the problematic disconnect between head and heart, that knowing isn't the same as believing, and that sometimes even the desire to believe doesn't translate into the kind of belief that calms our fears.
First, I think I'd tell him that he's not alone. Jesus' own disciples had trouble believing, Christian greats like Martin Luther and D. Martin-Lloyd Jones wrestled to live in faith (hence MLJ's great exhortation to constantly preach the Gospel to yourself), mommy and daddy often lack faith, and if we actually "get it," we are prone to forget and so Scripture is replete with exhortations to remember what God has done and to regularly remind one another.
Amazingly, there is One other who knows what it's like to struggle to believe, but he always ended up on the side of faith. But the book of Hebrews tells us that in the experience of struggling to believe, Jesus gained empathy with us; can you imagine Jesus struggling to believe God's promises as he was in the wilderness being tempted? As he was in the garden and going to the cross?
Here's what's great: God's promises don't depend on the strength of our belief. Yes, we take hold of them by faith, but once we've placed even the tiniest bit of faith in Christ, the strength of his overcoming faith prevails. The promises are ours no matter how little our faith; "Made like him, like him we rise; ours the cross, the grave, the skies!"
Christ carries us and the Holy Spirit intercedes on our behalf; God has made remarkable provision for the weakness of our faith.
I can't forget, though, to encourage T to ask God to increase his faith. If someone doesn't experience doubts about God, I'm not sure they are being entirely honest with themselves; but if they remain in doubt, they are placing the veracity of their own thoughts above those of God. It's okay to struggle to believe; it's not okay to glorify doubt as some sort of intellectually or spiritually superior state of being.
Sometimes (most of the time?) growth in grace is incremental and hard-won. Jacob wrestled with God and would not let go until God blessed him. The judge in Jesus' parable gave the widow justice because of her persistence, and the friend was given bread so the donor could get back to sleep.
T won't have to wait until he's 23 to know the words of Mark 9:24; it's a constant prayer for each of us. Lord, I believe; help my unbelief.