When I was in seminary I took a required course on preaching. In one of our class sessions the professor posed this question to all of us for response (and I hope you’ll chime in down in the comment section too!):
“Should we preach the gospel in every sermon, or should every sermon contain the gospel?”
During the course of our dialogue, someone in the room referenced the traditional image of a preacher ending a sermon with something like,
“With every head bowed, and every eye closed, would anyone like to receive Jesus Christ as your personal Lord and savior today?”
This, it is supposed by some, is an example of ending a sermon with “the Gospel.”
Other typical elements in this approach include exhortations to repent from the sins we’re sorry for, and agreeing with the fact that Jesus “died on the cross for our sins,” concluding with something like, “so accept Jesus now, and ask him into your heart. If you do this, you will be saved and assured of heaven when you die.”
Concluding elements in this understanding of “preaching the gospel” are things like raising hands so that the preacher can see you, saying a “sinners prayer” to ask Jesus to save you and be Lord of your life, coming to the front of the auditorium to stand with all the other people who accepted Jesus, and filling out response cards to be turned in at the end of the service so that a pastor can follow up.
This is variously referred to as “an altar call,” or a “salvation call,” or “casting the net,” or “an opportunity to make a decision for Christ.” All of this verbiage has come to be associated with “preaching the Gospel,” and it is what is behind the original question, “Should every sermon contain the Gospel?”
Some pastors pride themselves in ending every sermon with “the Gospel” (by which they mean something like what I described above), and think that not doing something like this is a pastoral failure.
Before I move on I want to say two things lest readers think that I am attacking this practice.
First, I have done something like what is described above more times than I can count over the course of my pastoral ministry and I have personally seen lots of people become followers of Jesus when I have done it. Some of them are still growing Christians who have become incredible leaders in the church I pastor, or have gone on following Jesus. One of the founding couples in our Church gave their lives to Jesus Christ at a Billy Graham crusade where what I described above was how they responded to Jesus – so this is not an attack on salvation calls or altar calls. I am sure I’ll use this ministry tool again and again in the future. But —
Second…what I described at the beginning of the post is not the same thing as preaching the Gospel. It is rather a “salvation appeal” based on an understanding of the gospel as a call to “get saved” so that a person can go to heaven after they die. Space doesn’t permit me to completely describe the difference between preaching the gospel and making a salvation appeal here, so I refer you to Scot McKnight’s book, The King Jesus Gospel to fill in all the blanks. In summary, McKnight helpfully differentiates between a “plan of Salvation” and “the Gospel,” which he says are two different things that have come to be thought of as the same thing. I agree.
In his book, Paul in Fresh Perspective, N.T. Wright provides the following excellent summary of The Gospel.
“the gospel [is the] royal announcement made by Jesus’ apostolic heralds” in which “God has declared in advance that he has dealt with sin and death, and has summoned the world to the obedience of faith, with the corollary that all those who believe find themselves declared in advance, as part of the apocalyptic unveiling of the ultimate future, to be within God’s true family, whether they be Jew or Gentile.” 
According to Wright’s summary of the Gospel, it is a royal announcement that Jesus is King, and is to be obeyed as King. For those who accept Jesus on these terms, they enter into the present work of God happening through Jesus, and are also promised that they will share in the eventual and climactic results of where that project is ultimately headed (Jesus’ resurrection being the precursor to what God has in mind). In short, Wright seems to be saying that the Gospel is “Jesus is King – obey Jesus.” My short summary of Wrights longer summary is, I believe, what is meant by The Gospel.
Okay, back to the question, “Should every sermon contain the Gospel?” If I use Wright’s long summary, or if I use my short summary, I would say… absolutely, yes! But this means that I may not often do altar calls for people to “get saved,” but rather I will often (and always) proclaim the Lordship of Jesus Christ in everything I preach, and exhort the listener to follow Jesus. In that sense, every Christian will need to hear the Gospel each time the scriptures are opened. I mean, every sermon should include a Christ-centered explanation of whatever text is being preached, and should include an evangelistic (and by this, I mean a royal summons and proclamation) call to follow Jesus.
This approach to preaching the Gospel is, I believe, much closer to what is meant by proclaiming Jesus is Lord. It is also the foundation for what Jesus said in Mat. 28:18-20. In that text, Jesus affirms that he has been given complete authority from God to rule over all of God’s creation, and that his followers were to bring all the nations of the world into that reality by baptizing them and teaching them to “obey everything I have commanded you.”
So, yes. Every sermon should contain the Gospel. But that doesn’t mean every sermon should end with a salvation appeal. Rather, every sermon should include the announcement that Jesus is Lord and the call to follow (e.g., obey) Him as the legitimate King. As a “gospel preacher” I can (and must) do this whether I am preaching a text about marriage, parenting, prayer, spiritual warfare, church life, leadership, fasting, finances, or witnessing (or whatever else).
So – Jesus is Lord. Follow Jesus!
 Wright, N.T. (2005) Paul in Fresh Perspective. SPCK, London (p. 57).