This is an idea which we find expressed many different times in the Course. The Course makes it clear that there is no holiness in this world, only the reflection of holiness. Love belongs to God. In this world we find not love but the reflection of love, which is forgiveness. So the goal of the Course is to teach us how to forgive, not how to love.
Forgiveness undoes all the obstacles or barriers to love.
Eternity is not possible in this world. But what foreshadows Heaven, what leads us to Heaven, is the forgiveness that is expressed in a holy relationship. The Course also talks about the holy instant, referring to the instant in which we choose the Holy Spirit instead of the ego, which is the reflection of the Holiness of God. One of the key concepts in the Course is the real world . It is not Heaven.