Today, it looks like the Gospel does not say too much about Jesus or about us. «Why do the people of this present time ask for a sign?» (Mk 8:12). Commenting on this episode of Jesus' life, Pope John Paul II says: «Jesus invites us to discern between the words and the deeds testimony of (or a “sign of”) the arrival of God's Kingdom». It seems the Pharisees questioning Jesus are lacking the capacity or the will to discern that sign which —in fact— is the entire manifestation, deeds and words, of the Lord.
Nowadays, Jesus is also asked for some heavenly signs: that He let us see his presence in our world or that He tells us in a clear cut manner how we are to behave. The Roman Pontiff makes us see that Jesus' negative to give a sign to the Jews —and, consequently, to us, too— is due to the fact He wants to change the «world logics, oriented to look for signs confirming man's desire of assertiveness and power». The Pharisees did not want just any sign, but one showing Jesus as the Messiah they wanted. They were not waiting for the Messiah coming to save them, but for the Messiah who was to give them the certainty they were doing things the right way.
In short, when the Jews in Jesus' time, or to-day's Christians, ask —one way or another— for a sign, what we are actually asking for, is for God to act according to our own way, that which we think is better and which also stands by our way of thinking. But God, who is omniscient and omnipotent (this is why in The Lord's Prayer we say “your will be done”), has His own ways which, more often than not, we find it difficult to understand. But He, who allows us to find him when we are truly looking for him, if we beg him to enlighten us, He will give us to understand which are his ways and how we can, today, distinguish his signs.