After graduating he undertook further studies at both Jena and Marburg in Germany and then went to teach in Canada and the United States.

But his most important contribution to theology was an exploration of the relationship of the knowledge of God to spiritual and moral experience.

He served alongside of John T. McNeill and Henry P. Van Dusen as a general editor of the Library of Christian Classics series, which includes modern translations of the writings of Christian theologians and thinkers such as Aquinas, Augustine, Calvin, Luther and other reformers and early church fathers.

[7] As Convener of the Church of Scotland's General Assembly's "Commission for the Interpretation of God's Will in the Present Crisis" ("The Baillie Commission"), reporting to the Assembly 1941 to 1945, Baillie helped the Church to think through its approach and mission to the post-war world.

[9] Shortly after his death in 1960, the series of Gifford Lectures he had prepared for the 1961–2 academic year was read by John McIntyre and Thomas F. Torrance and published by Oxford University Press.