Bone is the sole biological material found in the human body that is able to sustain compressive loads. However, although the structure of bone is well-known (it is a natural composite of collagen protein and hydroxyapatite mineral with a complex hierarchical organization), the details about the mechanisms that govern deformation at the molecular scale under compressive loading are still not completely understood. To investigate the molecular origins of bone's unique compressive properties, we perform full atomistic simulations of the three-dimensional molecular structure of a mineralized collagen fibril, focusing on the role of intrafibrillar mineral densities in dictating the mechanical performance under compressive loading. We find that as the mineral density increases, the compressive modulus of the mineralized collagen increases monotonically and well beyond that of pure collagen fibrils. These findings reveal the mechanism by which bone is able to achieve superior load bearing characteristics beyond its individual constituents. Moreover, we find that intrafibrillar mineralization leads to compressive moduli that are one order of magnitude lower than the macroscale modulus of bone, indicating that extrafibrillar mineralization is mandatory for providing the load bearing properties of bone, consistent with recent experimental observations.