God has revealed Himself in Scripture—as one God eternally existing in three Persons.
Jonathan Edwards noted, after studying the topic, "I think [the doctrine of the Trinity] to be the highest and deepest of all Divine mysteries"
Part one uses an analogy by C.S. Lewis in "Mere Chrisitianity"
Part two deals with problem analogies and the historical background of the doctrine.
For further study, see James R. White, The Forgotten Trinity (Minneapolis: Bethany House, 2001).
or use these online links:
An Unpublished Essay on the Trinity by Jonathan Edwards
"Hear, O Israel: The LORD our God, the LORD is one."
"Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit."