No, Einstein's theory of relativity has not been used to determine the historical existence of Jesus. The theory of relativity is a scientific framework that deals with the nature of space, time, and gravity, and its applications are primarily focused on physics and cosmology.
Determining the historical existence of Jesus is a topic that falls within the realm of historical research, archaeology, and religious studies. Scholars and researchers in these fields employ various methodologies, including textual analysis, historical context, archaeological evidence, and comparative studies to study the life and existence of historical figures.
Einstein's theory of relativity, while groundbreaking in the field of physics, does not provide direct evidence or tools to investigate historical or religious claims. The theory deals with the fundamental principles governing the behavior of space and time, not matters of historical or religious inquiry.
Therefore, the question of Jesus' existence is a matter of historical analysis and is approached through different scholarly disciplines, independent of Einstein's theory of relativity.