First, God put the tree of the knowledge of good and evil in the Garden so that Adam would have to live antithetically, saying “Yes” to God and “No” to sin. God receives great glory when man lives antithetically. If the tree of the knowledge of good and evil was not in the Garden, then Adam would never have had to say “No” to sin. There would have been nothing bad in the Garden. But, with the tree of knowledge there along with the good trees, Adam would have to reject the wrong and do the good. God is greatly honored when man lives an antithetical life.
Second, God put the tree of knowledge in the Garden because His plan all along was to save His people in Christ. He planned for Adam to sin so that His Son would come to deliver His people. He determined that this would give Him greatest glory since it would show forth His saving power. A passage that shows God eternally planned for Christ to come for His glory is Eph. 1:9-12. God also planned Adam’s fall ultimately for our good. In Jesus Christ, we will be perfected and live with God forever in a friendship even greater than Adam had with God.