The Elements were a spiritual belief that was shared by the Romulan people.
The concept states that there are no deities but instead asks for reverence to be displayed to the Elements that make up the universe itself. These elements are: Fire, Earth, Water, Air and the Archelement which oversees the others. (TOS - Rihannsu novel: My Enemy, My Ally)
They were often called upon in times of fear or uncertainty. Romulans were also known to take them in vain on occasion, as "By the Elements!" was sometimes used as an exclamation of surprise. (TOS - Rihannsu novel: The Romulan Way, STO mission: "New Romulus: The Power Source")
The idea of the Elements originated during the early years of the Sundering as a discussion board joke among the proto-Romulans on the Vulcan equivalent of the Internet. The joke proved popular and enduring enough that various idioms began to derive from it, such as that a particularly stubborn person "had too much Earth in him". Even S'task took part in these discussions on occasion.
The fact that this was a joke was gradually forgotten, and by the time the survivors of the Romulan colony fleet reached Romulus it had become the primary spirituality of the Romulan race. (TOS - Rihannsu novel: The Romulan Way)