Jews wanted signs and Greeks wanted wisdom, and the cross demands faith. 1 Corinthians 1:20-25 continues to talk about the foolishness of man's wisdom. People today want these same things before they will believe in Christ. Some people want God to give them a sign of His power and existence. Others want scientific proof of God or something they can do to earn salvation. Hebrews 11:1 tells us, "Now faith is the assurance of things hoped for, the conviction of things not seen." This is how the people of the old testament were saved too (keep reading in Hebrews 11), prior to Christ's death. They had faith in the coming payment for their sins.
The power of Christ crucified is a stumbling block for some and folly to others, but to those who are called, "Christ the power of God and the wisdom of God." Really, if we could figure it all out on our own, we would be taking the power away of God, and we would not need God. But "the foolishness of God is wiser than men" (ALL MEN COMBINE), and "the weakness of God is stronger than men" (ALL MEN COMBINE). If God had a weak point in His being, it would still be stronger than all of the strength of mankind. All of our creations as mankind, don't add up the the complexity of one grain of matter. This reminds me of a joke:
One day a group of scientists got together and decided that man had come a long way and no longer needed God. So they picked one scientist to go and tell Him that they were done with Him.
The scientist walked up to God and said, "God, we've decided that we no longer need you. We're to the point that we can clone people and do many miraculous things, so why don't you just go on and get lost."
God listened very patiently and kindly to the man and after the scientist was done talking, God said, "Very well, how about this, let's say we have a man making contest." To which the scientist replied, "OK, great!"
But God added, "Now, we're going to do this just like I did back in the old days with Adam."
The scientist said, "Sure, no problem" and bent down and grabbed himself a handful of dirt.
God just looked at him and said, "No, no, no. You go get your own dirt!"