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How did Albert Einstein connect acceleration and gravity in his thought experiment?

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Portrait of Albert Einstein in a suit and tie, looking slightly to the side with a neutral expression. — Albert Einstein Albert Einstein developed the special and general theory of relativity, which recast gravity as a bending in space-time.

Albert Einstein connected acceleration and gravity through his principle of equivalence, a concept he developed using a famous thought experiment involving an elevator.12

Einstein realized that a person inside a sealed, freely falling elevator would feel weightless because both the person and the elevator accelerate downward at the same rate. Within this system, it is impossible to distinguish between the effects of gravity and those of acceleration; for instance, if a ball were released, it would appear to remain stationary relative to the observer, just as it would in the absence of a gravitational field.12

He extended this logic to light. If an elevator is accelerated upward, a beam of light entering through a hole in one wall would appear to follow a curved path as the elevator moves. By the principle of equivalence, if acceleration causes light to bend, gravity must do the same. This insight led Einstein to conclude that gravity is not merely a force but a geometric phenomenon that curves the path of light and the structure of space-time.123