Sound The Trumpet

John 3:13-16 And no man hath ascended up to heaven, but he that came down from heaven, even the Son of man which is in heaven. (14) And as Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up: (15) That whosoever believeth in him should not perish, but have eternal life. (16) For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. .

Jesus refers to Numbers 21. Because the people were speaking against God, God sent serpents in among the people. Anyone who was bitten by one of these serpents would die. God, in His mercy, gave them an escape. God instructed Moses to hold a brass serpent (snake) up on a pole. When someone was bitten, all they had to do was look at one of these serpents and they would be healed.
Jesus explained that He would be lifted up just as Moses lifted up a serpent. Jesus went on to explain that anyone, no matter who they were, could believe in Jesus and be saved.