We know what has been written, we doubt what has been changed but we ignore what has been forgotten.
The word of God is so important for the human being, it helps the person follow certain rules which will keep him on the right path and live a good life while expecting the reward for his right conduct in the next one. The rules of God are higher than the ones we make. If we follow the guidelines He sent us, we will be happy and rewarded for our obedience. If we desobey being headless of His commandments, we will be punished in the hereafter. So simple, yet ignored.
The Lord sent us messages, warnings, knowledge, prophets, signs to help us see who He is and how we should behave in this temporary existence. Do you think it is thanks to you that the rivers flow? Do you think it is thanks to your knowledge that vegetables came out of the ground in the first place? Everything we have (from the sun light to the cup of tea on your table) is created by God. We have to acknowledge that so that we might be thankful.
God sent us His messngers/prophets, because He saw His creatures fighting, robbing, killing, taking advantage of the weak, hoarding wealth as if they were going to live forever, pushing away the needy…. He saw rebellion and greed in us. Deeds that rejoiced the devil and angered God.
But their contemporaries were ungrateful, they rejected their teachings. Some were beaten, tortuted, or even killed by those opposing them. Even though deep down, they knew that they were messengers of God, but jealousy, greed and pride pushed them to refuse submission to God. Still, the Lord sent others to bring them light and justice.
Abraham, Isaac, Jacob, Joseph, Élie, Job, Salomon, David, Jonas, Lot, Noe, Moses, Aaron, Jesus, Mohammed, they were all sent to teach us and guide us to the right path leading to Heaven.