J. Mack Stiles attempts to bring together two topics that are often separated in the minds of Christians. These ideas should be kept together.
Some authors emphasize content. They build on the truth that the gospel is the power of God for salvation to all who believe. If we get the content down, God can use it for His glory.
Other authors emphasize character. Think lifestyle evangelism. They downplay content and build upon the truth that if I speak without love I am just a resounding gong or a clanging symbol. They want Christians to walk the walk, even if they downplay talking the talk.
J. Mack Stiles does the rare thing of actually interacting with both.
Greatest StrengthThe greatest strength of this short book is its attempt to unite both message and messenger. Stiles attempts to emphasize the importance of actually preaching the gospel in its fullness while also emphasizing the character and conduct of the one speaking these truths.
In doing so, Stiles has brought together the full scope of teaching from the Scriptures that many authors miss because they are busy emphasizing one to the neglect of the other. For this effort, Stiles should be commended.
Greatest WeaknessDue to the attempt to navigate both the message and the messenger, while also keeping the length short and including a lot of anecdotes and illustrations, the weakness becomes evident that Stiles will not be the best exploration of content or character that you can find. Other authors who emphasize the importance of content only will delve much deeper. Likewise, those who emphasize only character will likewise find their treatments more exhaustive.
ConclusionIf you're looking for the best treatment of either the content of the gospel or the character of a mature Christian, this book will probably let you down.
If you're looking for an encouraging and edifying read that is balanced in its treatment of both content and character, then I think you will find it in Marks of the Messenger.
I would be confident recommending this book to any Christian I know who has been taking their call to proclaim the gospel seriously and who is committed to growing in Christ's likeness by the grace of God.