If the court finds [the thief] guilty, he shall pay twofold (Exodus 22:8).
The Talmud explains that an armed robber can make restitution simply by returning the stolen item. A thief, who steals in stealth, must return the object and pay a heavy fine.
Why is the thief punished more severely? By operating in stealth, he indicated that he feared being observed by humans, but was not concerned that God saw his deeds. In other words, he essentially denied the providence of God. While the robber's act was just as dishonest, he at least equated people with God, in that he operated in full view of both. Hence, although his attitude was one of defiance of God, it was not necessarily one of denial.
Sometimes we do things that are not ethically sound, and in order to avoid social sanction and to maintain a reputation of decency, we may act in such a manner that it appears to be ethically proper. While we may indeed succeed in this deception, we must remember that there is One Who cannot be deceived, and Who knows the truth of our behavior. We should realize that acting in such a manner is essentially a denial as well as a defiance of God.
It is evident now why the thief pays double. The robber pays only for defiance of God, whereas the thief must pay for defiance and denial.
People who think they are willing to sacrifice their very lives rather than deny God should reflect on whether they might not actually deny God for the sake of mere monetary gain. The acid test of loyalty to God is not just in martyrdom, but in living honestly.