For the Jews require a sign, and the Greeks seek after wisdom:
For - literally, Whereas. A B C Delta, Vulgate, add 'also,' taking up the "whereas" of 1 Corinthians 1:21, and additionally illustrating that 'the world knew not God.'
A sign. 'Aleph (') A B C Delta G, Vulgate, read 'signs.' The singular was a correction from Matthew 12:38; Matthew 16:1; John 2:18. The signs the Jews craved were not mere miracles, but direct tokens from heaven that Jesus was Messiah (Luke 11:16).
Greeks seek ... wisdom - namely, a philosophic demonstration of Christianity. Christ, instead of demonstrative proof, demands faith on the ground of His word, and of reasonable evidence that the alleged revelation is His word. Christianity begins not with solving intellectual difficulties, but with satisfying the heart that longs for forgiveness. Hence, not the refined Greeks, but the theocratic Jews, were the chosen organ for propagating revelation. Again, intellectual Athens (Acts 17:18-21) received the Gospel less readily than commercial Corinth.