Historic and Confessional References
For a true understanding of the doctrine of the Trinity three questions must be answered:
What is the meaning of the word "essence"? What is meant by the word "person"? And what is the relation etween "essence" and "person" and between the persons among themselves?
The divine nature cannot be conceived as an abstract generic concept, nor does it exist as a substance outside of, above, and behind the divine persons. It exists in the divine persons and it totally and quantitatively the same in each person. The persons, though distinct, are not separate. They are the same in essence, one in essence, and the same being. They are not separated by time or space or anything else. They all share in the same divine nature and perfections. It is one and the same divine nature that exists in each person individually and in all of them collectively. Consequently, there is in God but one eternal, omnipotent, and omniscient being, having one mind, one will, and one power.
Bavinck, Reformed Dogmatics vol. 2 God and Creation, p. 298-300
By Scripture
Old Testament