(Eph. 6:19) ... the mystery of the Gospel.

This verse implies that there is some mystery in the Gospel.

By the very meaning of the word mystery this means that the Gospel is not what we think about it (but there is a mystery). In other words, Gospel is not straight for understanding (there is a hidden meaning that is a mystery, something we didn't expect in its meaning).

My book reveales the mystery of the Gospel. The mystery is that the Gospel cancels itself just like as it cancels Old Testament.

Without knowing this mystery, we cannot put different parts of the Gospel in accord, are spiritually blind and even fall into contradictions.