“One day some teachers of religious law and Pharisees came to Jesus and said, “Teacher, we want you to show us a miraculous sign to prove your authority.”” Matthew 12:38 NLT
A sign used in the Bible is for some event or activity, supernatural or not, that would authenticate the person and claims of Jesus. A sign was usually a miracle with a clear meaning; it was a miracle designed to reveal something specific. However, in the Bible there are two ways that signs are used, to convince and to confirm.
For example, when Moses was sent back to the Israelites to lead them out of bondage, he was given some signs to perform--his staff turning to a snake, his hand turning leprous, and the water turning to blood. These were done in order to convince the people that they should believe Moses and follow him. But Moses was given another sign--when he and the people returned to Mount Sinai after the exodus they would worship at the mountain. That was a sign that would confirm that God had done it, but it was not a sign to convince them to go to the mountain to worship. Once they got there they would be assured that God had done just as He had promised.
These Pharisees and Sadducees clearly wanted a sign to convince them to believe. But they were dishonest and Jesus saw right through them. They had just seen a spectacular sign, the casting out of the demon so that the man regained his abilities, and instead of believing they accused Him of doing it by Beelzebub. They were not interested in a sign, only in trying to discredit Jesus.
If He could not do a sign for them, they could expose Him; if He did one, they could discredit Him. They were an evil lot and they call Him “Teacher”! They despised Him and were determined not to listen to His teachings.
Moreover, wanting a sign runs contrary to the nature of faith which does not rely on a sign to convince people to believe. If Jesus did a sign like that, it is unlikely that these people would have believed. They were merely challenging Jesus, and if He did a sign they would likely have rejected it. After all, they had frequently explained away some of the great miracles He had been performing.
We should digress for a moment because there are places in the Bible where it seems appropriate to seek a sign from God. We have already seen how God gave Moses signs to do to authenticate His plan. The people needed to be sure that this man off the desert was truly sent by God.
When they saw the signs, that was enough--they accepted him as their leader. It was not a question of coming to faith in God, but rather of testing the authenticity of a man who wanted to lead them out of bondage.
Another good example; God through Isaiah told the king to ask a sign from the Lord, anything whatsoever (Isa. 7). The king refused, being a wicked unbeliever. So God gave a sign anyway: a virgin would conceive and give birth to a son known as Immanuel. The supernatural birth of Jesus would be a sign that the Davidic Covenant would still be fulfilled. The point in Isaiah 7:9 is that if Ahaz had believed, he would have been confirmed. So asking a sign in faith is different than challenging God to convince us to believe.
Or, Gideon put out the fleece for a sign that God would go with him to battle. The text never condemns Gideon for this, because he was a devout believer, but more importantly, he had already decided to go, and what he wanted was a sign that God would be with him. So again faith was already operative.