Lessons to learn about the appearance of Moses & Elijah and departed loved ones:
They lived on even though they had left earth hundreds of years before.
They could be identified without any formal introduction.
They could speak to each other.
Their intelligence was not impaired.
They knew about coming events.
They showed no dread in the presence of the glory of Christ.
They were able to commune with Christ (Mt 17:37).
The focus of their attention was upon the Cross (Revelation 4-5).