No other scientist may have had a greater impact on modern cosmology than the Belgian physicist, astronomer and priest Georges Lemaître. In 1927 he predicted the expansion of the universe on the basis of the cosmological field equations; and four years later he proposed what he called the primeval-atom hypothesis, the first version of the later big bang universe. In all his work on cosmology the cosmological constant Λ played a significant role. A recognized expert in the theory of general (...) relativity, Lemaître also contributed significantly to the theoretical clarification of local and global singularity problems. Still, when he died in 1968, at a time when the standard big bang model celebrated its first victories, he was largely forgotten or recalled only as a somewhat shadowy figure of the past. This essay reviews in a historical context the scientific work of Lemaître with particular attention to his seminal contributions in the decade between 1925 and 1934. (shrink)