That is a typically Pauline sentence, it goes on and on and on. So take it one piece at a time. He is talking to people who have now been accepted in Christ. When he talks about being in Christ, that is a code term which he uses for a Christian. Paul talks about being in Christ! Then he said, "You also are accepted in Christ, included in Christ when you heard the Word of Truth, the gospel of your salvation." So here are the people he's talking to. Some of them are Jewish people, who as yet have not heard that Jesus of Nazareth was shown to be the Messiah by the resurrection for the dead. They didn't know that. In addition to those Jewish people, there were also pagan people -- people from all over the places where Paul had been traveling, and he describes them as without God, without Christ, and without hope.
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To all these people the Apostle Paul has come with that he calls the Gospel of Salvation. Gospel simply means 'good news'. Salvation is all about God doing for us what we can't do for ourselves. We cannot make ourselves fit for Heaven; we cannot make ourselves right with God; we cannot live life on earth as we ought to live it. We can't do it! We are not wired up adequately to do it! We are 'fallen' creatures, but God has taken the initiative to move into our lives, and do for us what we cannot do for ourselves. He has moved to save us from a situation from which we cannot save ourselves, and He does it through Jesus! That’s the Gospel of Salvation.
When you hear the Gospel of Salvation, there are two responses: one is to say, "That's a lot of nonsense!" And the other is to say, "That is true!" So immediately, there's a huge division. You hear the Gospel of Salvation, some people will say, "It is a Word of Truth." Notice, what Paul says, "You were included in Christ when you heard the Word of Truth, the gospel of your salvation. But then he says, "Having believed!"
Step number one: Hear the Gospel. Step number tw Come to the conviction it is a Word of Truth. Step number three: You believe “into” the Gospel. Now this means more than just giving a mental assent, an academic nod; more than saying,"”Sure, I believe that's true.” To believe into means to trust in, or to commit to. You can go to an airport and somebody will say, "That's an airplane, if you get in the airplane, it will miraculously take you to where it's going on the other side of the world. You might say, “Wow! That absolutely true!” Some people say, "That's nonsense," and walk away. Other people say, "That's true,” and sit down and watch it; they don't move an inch. Then they'll say to you, "Now, you need to get on the plane, sit down, fasten your seatbelt." You believe into it! You commit yourself to that which you are convinced is true.