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Mark 8:11-12 The Pharisees came and began to question Jesus. To test him, they asked him for a sign from heaven. He sighed deeply and said, "Why does this generation ask for a miraculous sign? I tell you the truth, no sign will be given to it."
Matt. 16:4 A wicked and adulterous generation looks for a miraculous sign, but none will be given it except the sign of Jonah." Jesus then left them and went away.
John 20:30 Jesus did many other miraculous signs in the presence of his disciples, which are not recorded in this book.
Acts 2:22 "Men of Israel, listen to this: Jesus of Nazareth was a man accredited by God to you by miracles, wonders and signs, which God did among you through him, as you yourselves know."
In this set, Mark's passage in the supposed "odd man out". Is Mark to be set against these other verses?
First note the parallel in Matthew 16, in context:
He answered and said unto them, When it is evening, ye say, It will be fair weather: for the sky is red. And in the morning, It will be foul weather to day: for the sky is red and lowering. O ye hypocrites, ye can discern the face of the sky; but can ye not discern the signs of the times? A wicked and adulterous generation seeketh after a sign; and there shall no sign be given unto it, but the sign of the prophet Jonas. And he left them, and departed.
Mark is preserving a shorter version of this episode; does he mean to say that Jesus would not perform any miracles at all? Hardly, since he has Jesus doing them in plenty.
A key here is that this account comes right after the miraculous feeding of loaves and fishes -- in other words, Jesus has already performed a honking big sign, and the Pharisees, etc. don't accept it. They doubted not Jesus' ability to do signs, but their origin in God.
Note that the Pharisees ask for a sign from heaven (the words "from heaven" were often a circumlocution for "from God"; but this could just as well mean "from the sky" -- Keener, 420) -- not just any old sign.
This is the context of Jesus' refusal. The remaining verses are therefore not in contradiction.