Many churches fail to teach the Gospel clearly, or even at all. This failure is a major apostasy (false belief). The Gospel is either at the heart of Christianity, or even, from a more extreme viewpoint, actually is Christianity, so it certainly cannot be missing from any genuine Christian context. Those churches that do “expound” the Gospel often give it an unfortunate twist, so some explanation of it is in order here.
On the face of it the Gospel is incredibly simple, roughly equivalent to 1+2=3 in arithmetic. The problems occur in understanding the “One” and the “Two”. The One is about understanding oneself as a sinner. Many people who are not Christian lead good lives. They may never have paid much attention to the Ten Commandments, but none the less have followed most of them most of the time. This should not be a surprise if people are made in the image of God. However, there is a frequently forgotten Commandment that is probably also the Greatest: Jesus stated it as “Love the Lord your God with all your heart and with all your soul and with all your mind”. To observe this Command is especially difficult when the very concept of a Creator God is missing from people’s world-view, which inevitably it is in a society that accepts Darwinian Evolution. So the first point to grasp is that there is a Creator God, and that He is the one who decides what is right and wrong. The things that He deems to be wrong are called “Sin”, and this is something quite different from crime or the conventions of any particular society. Living your life with your back turned to God is living in Sin, whatever else you do or don’t do.
The second thing to understand is total forgiveness. As human beings, we find forgiving others difficult or impossible, so the idea that God would simply forgive us completely rather than punish us seems, at the very least, unlikely. And the mechanism that makes this possible does indeed seem bizarre. It is that all the punishment due to all people who ever offended God was placed upon Jesus when he was crucified; and God’s forgiveness is released to people just as soon as they conclude (with absolute sincerity) that they are a sinner in need of forgiveness, and that Jesus made that forgiveness not only possible, but guaranteed. This is not only bizarre, but also, from any logical standpoint, actually unfair! How can such a whacky idea, especially originating as it did with a troublemaker in a remote province of the Roman Empire, have ever gained any traction? How can it be that throughout the last two thousand years there have been people in every generation who have not only believed it, but been prepared to die for it? It can only be because it’s true, or because there are a lot of very stupid people about. We all, whether it be by default or conscious thinking, make a choice between those two conclusions.
Because many churches muddy the waters of the Gospel, it is necessary to be clear about what people need to do in order to receive the forgiveness the Gospel promises: it is simply a change of mind to line up with what the Gospel declares. It is not necessary to stop all wrongdoing, go to church, turn from addiction, or whatever other additional actions people may suggest. It is quite simply a change of mind. This does not mean, as some claim, that one can go on sinning. It is inevitable that a change of thinking will result in a change of actions. What the changes will be and when they take place will vary from person to person, but a process of change will inevitably begin with the change of heart.
There are plenty of other Gospel expositions on the web which you may wish to review, but the first and totally reliable source is, of course, the New Testament.