 Jesus harshly rebuked religious people in a specific instance when they demanded a sign. Jesus had already performed many miracles. Those who asked weren't sincerely asking for a sign in faith. They were cynically demanding one out of stubbornness. It's a mistake to use this specific instance as a basis to then say that we mustn't ask God for signs today. Jesus healed to prove he could forgive sins. If signs were evil, they wouldn't accompany the gospel. Hezekiah asked God for a sign and the Lord obliged. Gideon reverently asked God for two signs. Jesus told us to believe him based on his works. God has given us signs of his very existence. After the death of Lazarus, Jesus told his disciples that he was glad that they would see the resurrection of Lazarus because it would give them a stronger belief. God's willingness to give us signs continues in the book of Acts. God has not stopped interacting with man. And that interaction, by its very nature, brings forth signs.