The “tax” in question was the Roman census tax, a head tax levied on Judea since AD 6, when Rome took direct control of the province; it had already sparked a revolt under Judas the Galilean (Acts 5:37 remembers it). Paying it was the daily, hated proof of foreign rule.

The denarius they handed Him was a day's wage, and the standard silver denarius of Tiberius carried the emperor's portrait with the Latin inscription “Tiberius Caesar, son of the divine Augustus.” That is why the question was a trap with no safe answer: say “pay,” and He endorses a coin that hails a man as a god's son; say “don't,” and He is a rebel against Rome. His answer, “whose image?”, turns the coin's own claim back on it: the coin bears Caesar's image, but man bears God's.