The Einstein formula: $E_0=mc^2$. “Isn't the Lord laughing”?

The article traces the way Einstein formulated the relation between energy and mass in his work from 1905 to 1955. Einstein emphasized quite often that the mass \it{m} of a body is equivalent to its rest energy $E_0$. At the same time, he frequently resorted to the less clear-cut statement of the equivalence of energy and mass. As a result, Einstein's formula $E_0=mc^2$ still remains much less known than its popular form, $E=mc^2$, in which $E$ is the total energy equal to the sum of the rest energy and the kinetic energy of a freely moving body. One of the consequences of this is the widespread fallacy that the mass of a body increases when its velocity increases and even that this is an experimental fact. As wrote the playwright A. N. Ostrovsky “Something must exist for people, something so austere, so lofty, so sacrosanct that it would make profaning it unthinkable”.