God's eternal purpose is to express Himself fully in and through man. He created Adam for this purpose, but Adam fell. Abel was brought back to God by offering the proper sacrifices. After those two landmarks, Enosh, Enoch, Noah, Abraham, Isaac, Jacob, and Joseph followed as individuals to express God. Yet what God wanted was not an expression through individuals only. He wanted a corporate expression. So after Joseph died, in Exodus God gained a collective people to express Him corporately. This people was eventually built up together to be His dwelling place. The sign of this was the tabernacle which was God's dwelling place on this earth. First it was a tabernacle, and eventually it became a temple. If you pick up these nine great men plus the tabernacle and temple you cover the entire Old Testament.
The New Testament began with a person, an individual. This individual is a marvelous, wonderful, and all-inclusive Person who is both God and man. That is Jesus Christ. He came to dwell among man as a tabernacle to declare the invisible, unseen God (John 1:14, 18). The unseen, invisible God has been fully expressed in this man, Jesus Christ, as God's tabernacle among men. But again God was not satisfied with just one man as a tabernacle. God wanted a collective people to be His tabernacle. So the Lord Jesus told His disciples that He was going to die and be raised up that He might be enlarged and increased (John 12:24). The way for Him to be enlarged and increased was to change in form. When He was the tabernacle on this earth among man, He was in the form of flesh. But in the flesh He had no way to enter into His believers so that they could be His increase. In order to enter into so many believers, He needed to change His form from the flesh to the Spirit.
In John 1, the One who was the Word of God, which was God Himself, became flesh. This is so mysterious! He became a material flesh, which is visible and touchable. But after His death and resurrection He came back to His disciples in another form. You may say that He came back in the form of the Spirit, but He also had a spiritual body. He showed His disciples His hands and His side (John 20:20), indicating clearly that after resurrection He still had a body, but not in the natural form. It was not a natural body, but a spiritual body.
The Apostle Paul in 1 Corinthians 15:35-38 illustrated such a change in form, such a transformation, by a seed. A seed has a form, and it may be yellow or brown, and round like a little ball. If you sow this seed into the earth it dies there. Then it rises up, or it grows up. To rise up is to be resurrected from the dead. When this seed grows up, the form changes. It still has a form, but it changes from a little round yellow or brown ball into the form of a sprout, so tender and green and lovable. It is still the same seed, but in another form. This is marvelous! When it gets into this changed form, it grows and also produces. It increases from one grain to thirty or sixty or a hundred.
This is a good illustration of the Lord Jesus being crucified, being buried, and rising up. When He rose up He was no more in the original form. He was no more merely an individual grain. John 12:24 says that He was the one grain which fell into the earth. But after rising up He was no more a single grain. He rose up to grow and produce many grains. Who are the many grains? They are all the members of His Body as His increase. This is mysterious, yet we can see it in nature all the time. In the natural things such as a grain falling into the earth and rising up, we can see such a mystery. Before falling into the earth it is one grain, but after falling into the earth and rising up, it becomes many grains. The many grains are the increase of the original grain.