...I can suggest some possible reasons that make sense to me as to why God did not immediately send Christ. They are all related to the fact that we learn within time. As creatures we are bound to time and space. What God did immediately do was to promise He would save fallen man. Genesis 3:15 is the beginning of the gospel announcing that someone born of a woman (we now know it was Jesus) would ultimately triumph over Satan: “And I will put enmity between you and the woman, And between your seed and her seed; He shall bruise you on the head, And you shall bruise him on the heel.” Satan thought he crippled [bruised heel] Jesus on the cross, but the resurrection fatally wounded [bruised head] Satan’s power. This is the first of many promises that God made; and, every one He has faithfully kept. “So shall My word be which goes forth from My mouth; It shall not return to Me empty, Without accomplishing what I desire, And without succeeding in the matter for which I sent it” (Isaiah 55:11). By God waiting we learn more about His faithfulness. Since He always does what He promises, we learn more about His truthfulness. “God is not a man, that He should lie, Nor a son of man, that He should repent; Has He said, and will He not do it? Or has He spoken, and will He not make it good?” (Numbers 23:19).
Jesus was the Lamb, a sacrifice, the Passover, Priest, Prophet, King, the fulfillment of the law, the Bread of Life, the Word. Without time to develop these ideas throughout history and through the writings of the Scriptures, we would never have understood the depths of the Godhead. In fact, God knew the exact second when He intended to send Christ to the earth. Galatians 4:4-5 is a beautiful statement of this: “But when the fullness of the time came, God sent forth His Son, born of a woman, born under the Law, in order that He might redeem those who were under the Law, that we might receive the adoption as sons.” He did not come earlier in order that we might learn faith. “But before faith came, we were kept in custody under the law, being shut up to the faith which was later to be revealed. Therefore the Law has become our tutor to lead us to Christ, that we may be justified by faith” (Galatians 3:23-24).
In the same manner, we learn about longsuffering, glory, and mercy because God chose to wait-”What if God, willing to shew his wrath, and to make his power known, endured with much longsuffering the vessels of wrath fitted to destruction: And that he might make known the riches of his glory on the vessels of mercy, which he had afore prepared unto glory” (Romans 9:22-23)....
It is an amazing and wonderful thing to realize that God devised His plan, created us, and is perfectly executing the plan, so that we could enjoy Him. He was infinitely satisfied within the Godhead before creation. He didn’t create for Himself; rather, at His pleasure He created for us. Just to have lived is a gift beyond measure. That God would allow some of His creatures to live for eternity in a relationship with Him is beyond comprehension. I will not reject it just because I cannot understand it. “But God, being rich in mercy, because of His great love with which He loved us, even when we were dead in our transgressions, made us alive together with Christ (by grace you have been saved), and raised us up with Him, and seated us with Him in the heavenly places, in Christ Jesus, in order that in the ages to come He might show the surpassing riches of His grace in kindness toward us in Christ Jesus” (Ephesians 2:4-7).
Taken from "Why didn’t God send Jesus to the earth to die right after Adam and Eve sinned?" by Randy Alcorn, Eternal Perspective Ministries, 39085 Pioneer Blvd., Suite 206, Sandy, OR 97055, 503-668-5200, www.epm.org