1 Corinthians 1:23 (AV)
23 But we preach Christ crucified, unto the Jews a stumblingblock, and unto the Greeks foolishness;
Signs and wisdom will not save (see verse 22). The cross of Christ is the theme of the Gospel. It is the heart of the Gospel. Rather than signs and wisdom, We preach Christ crucified, and this message is a stumblingblock for the Jews. How could the Messiah have been crucified? It did not make sense to the Jews since they were expecting Messiah to be their King and release them from their bondage. Jesus could not have been the Messiah if He was crucified. The Jews simply did not get their theology right.
This idea of a stumblingblock was prophesied back in Isaiah 8:14.
Isaiah 8:14 (AV)
14 And he shall be for a sanctuary; but for a stone of stumbling and for a rock of offence to both the houses of Israel, for a gin (trap) and for a snare to the inhabitants of Jerusalem.
Later, Isaiah prophesied that the Messiah would be a suffering Servant. They should have put the two things together; suffering Servant and King. Bad theology trips people up. This trips up the Jews.
From the Greek’s standpoint it was silly. Someone who proclaimed to be a god…crucified? To them, this religion was simply folly.
Today, unbelievers are tripped up by the preaching of the Gospel, and claim it is foolishness.