One of Hengel's main points is that the gospels were written with titles already attached to them. "The gospel according to X" is how they were known from the very time of their composition. This view is in contrast to the view of some scholars who claim the gospels circulated as completely anonymous works until the mid to late 2nd century. Hengel's arguments on this topic are very persuasive. If the gospels were anonymous until the middle of the 2nd century, then we would not have expected such authors as Mark and Luke to be attributed to two of the gospels instead of the more respected Peter and Paul. There is also no known deviation in the naming of the gospels. This would seemingly be impossible if the titles were not attached to the gospels until 50-100 years after they began circulating because there was no centralized church government to impose such an edict in the second century. There is also no hint of any dispute in the early church regarding the titles of the four gospels.
Hengel frequently discusses the testimony of the early church Fathers such as Papias, Justin Martyr, and Irenaeus in order to reach his conclusions. Before assuming Hengel is another Christian apologist it should be mentioned that he does not believe Matthew or John were written by those two disciples. However, he does believe they were originally written with the titles "gospel according to Matthew/John". He dates Matthew to 90-100 C.E. and John to 100-110 C.E, while dating Luke to 75-80 C.E. He also believes Matthew used Luke as a source.
This is the type of book in which the reader will not agree with everything the author claims, but will most likely find some areas of agreement. Hengel's arguments are always his own and he isn't one to follow the "party line" on any particular issue. He is to be commended for his originality and willingness to think for himself rather than relying on constant appeals to previous scholars to support his views.