How can I grasp why one seemingly small first sin brought such extreme consequences on all mankind?
A viewer accepts on faith that death and all suffering came through one man's sin (Romans 5:12) and that Christ's act of righteousness was necessary (Romans 5:18), but asks how he can internally comprehend how one seemingly small sin led to such extreme consequences.
Answer
The best way is to look closely at the dialogue the devil had with Eve, which shows it was not a small sin but probably the biggest sin one can commit: maliciously usurping God, His world, and His whole plan so that He cannot have it. The temptation was that when you eat this fruit you will become just like God, which entails no longer doing what God wants, being fruitful and multiplying, and instead going your own way as your own god. God, who is a personal God of love and joy, wanted to create creatures to share that with, and He would have richly rewarded Adam and Eve for obeying, but bearing children and tilling the ground were laborious, and the devil offered them godhood without any of it, and forever to boot. So the temptation was not really about a piece of fruit; it could have been anything God forbade, because the temptation was, 'I can do away with God by becoming my own god.' That is the perennial problem of man: he does not want to listen to God's way of doing things, which seems too long or too laborious, so he wants the quick way, right now, which is impatience and evil. God's way was to create the world first, populate it over a long time, and then bring us to a consummation where we become like Him, which is what heaven will be; the devil offered it instantly, shoving God out the door. So it is not a small thing; it is contempt for God, and that is why it brought turmoil to the whole world.