The following quote is taken from The Messiah in the First Century: A Review Article by I. Howard Marshall, published in the Criswell Theological Review 1993. Marshall is making reference to a portion of an article written by Martin Hengel.
M. Hengel (“Christological Titles in Early Christianity,” 425-48) discusses a number of christological hymns and argues from them that early Christian christological thinking “was much more unified in its basic structure than New Testament research, in part at least, has maintained” (443). He holds that “Christ” figures as a title—“the Messiah died for our sins” in the formula in 1 Cor 15:3-5. “Son of Man” is regarded as a cypher rather than a title; the resurrection appearances confirmed the identity of Jesus as the heavenly Son of Man and led to his being recognized as the Son of God. The development of a high christology was completed within about 15 years.
When Hengel says “within about 15 years,” he means within about 15 years of Christ’s resurrection. This would have been at least several years before the earliest of the New Testament documents were written (the earliest being considered as 1 Thessalonians ca. 49-50 CE).
Marshall’s article was reviewing the symposium chaired by James H. Charlesworth at Princeton Theological Seminary on certain aspects of Judaism and Christian Origins.