The letter represents the doctrine of God's plan, the idea that God works with us, through everything He sends into our lives, to make us the people that He wants us to be with Him in heaven.
The letter is a good representation of this doctrine because: 1) a letter is sent, and because 2) a letter needs to be unfolded. Step by step, all that was in the envelope from the beginning needs to come to light. This is just like God's plan which was present from the beginning and it is still being unfolded, day by day, minute by minute, throughout the world.
God's plan is also like an envelope we unfold because its contents are often surprising. But what happens is never surprising to God. He knows what He put in action when He made the universe. He knows that He sent us the experiences we need to go through so that we will become the seeds of the people He wants us to be, seeds He will transform when our work on earth is over and we are brought to the full life of heaven.
We know that this is true because we cannot imagine that it is not. So much of what makes us who we are is nothing we have chosen. No one chooses their parents, their birthday, the circumstances of their birth, and countless other crucial experiences. Certainly God has not left us here at the mercy of fortune. Certainly when He made the world He knew what was going to happen.
When we believe in God's plan, we receive the most wonderful gifts of all:
Love of self. Because we realize that God has made us who we are.
Peace with anything we cannot change. Because we realize that whatever cannot be changed happened for God's good reasons.
Freedom from fear and pressure. Because we know that everything that is going to happen will also follow God's plan.