When Israel murmured against God about going back to Egypt God sent the symbol of Egypt (“fiery” serpent) to curse and remind them that there is no protection or salvation in Egypt. But all of this was just pointing to something greater, namely, The Curse that was lifted up on a pole at Golgotha named Jesus.
The gospel message is foolishness to men, not because it’s irrational but because it’s unlike any other message in the world. God chose a foolish message and a foolish means in order to keep humans from boasting in themselves. If you are going to follow Christ it will cost you your pride.