On occasion when I've tried to convince people to submit their lives to Jesus Christ and trust Him alone for salvation, I have heard, "I want more proof." I used to feel defeated by this request because I couldn't conjure up a miracle to convince them. Today, however, I take solace that Jesus Christ, himself, received demands for more proof (Matthew 12:38).
Jesus condemns the 1st century doubters and offers three shocking reasons why:
1) The sign of Jonah should be sufficient (Mt. 12:39-41): Like Jonah's three day, death-like jaunt in the belly of a great fish followed by a second chance for life (see Jonah 1-3), Jesus would spent 3 days dead in the belly of the earth and resurrect victorious over sin, death, and hell. This is the one sure sign that Jesus is the Christ and that all doubting should be stopped, all mouths hushed (Isa. 52:15).
2) The presence of one greater than Solomon should be sufficient (Mt. 12:42): But if that's not enough, these doubters should be shamed that a pagan queen (the Queen of Sheba) at least recognized God's presence and blessing during the life of King Solomon. If a pagan queen can recognize when God is at work, shouldn't the supposed children of God see that God is even more fundamentally at work in the person and work of Jesus?
3) A sign performed for the half-hearted will only make matters worse (Mt. 12:43-45): Finally, Jesus says that a sign done on behalf of a half-hearted believer is like a demon exorcism on someone not really ready to let the demon go. Eventually the person will be overpowered again by evil, and the consequences far worse.
The conclusion Jesus is trying to draw is simply this: Neutrality with regard to Jesus is not possible (see DA Carson's Expositors commentary). There is no option for people to give him half-hearted commitments. We either trust fully that Jesus is the Son of God who came to die for the sins of the world or we trust in other things. We either accept the revelation that Jesus came and conquered death or we suffer the consequences for demanding more. Allegiance to Jesus is non-negotiable.