THE JERUSALEM COUNCIL 4 DECREES pt1 Why did Jesus have to return for 40 days?

Why did Jesus have to return and teach the disciples THINGS concerning the kingdom of God for 40 days and 40 nights in HIS Resurrected body? What was there that he could not teach them in the 3 1/2 years that He had to return and teach them ? These truths are very important for you to understand in order to understand the New Covenant.