A small word can make a big difference to a sentence. Paul states that the person who calls on the Lord believes in him before doing so. It is possible for a person to believe about the Lord, but that is not the same as believing in him. We can describe a belief about Jesus as a notional faith and it can include an understanding that all that is said about him is true. But such a faith by itself is not saving faith, although it is part of saving faith. To believe in Jesus means to depend upon Jesus. Such faith is not detached from him or merely clinical in its assessment. Instead there is dependence upon Jesus and delight in him. True faith is that of a warm personal relationship with Jesus. When a person has that kind of faith, he will call upon the name of the Lord.
Paul uses different words to describe the necessary information that a person needs to be told in order for him to have faith. In verse 16 he calls it the gospel and in verse 17 he calls it the word of Christ. The first describes its nature (good news) and the second describes its content (truths about Jesus). Both the nature and content are always present when the message is declared accurately. If a message about Jesus does not contain good news it has not been told fully or properly.
Obviously, a lot could be said about the details of Jesus that are good news. In a sense, this is what Paul has done already in this letter to the Romans, and we can extend this to include the contents of the whole Bible. For now, we can say that the message is good news because it is about salvation from sin and its consequences, and there is salvation because Jesus provided it through his activities on behalf of sinners.
It is important to note that Paul insists that the gospel is a message with authority. We can see that from his comment in verse 16 that many have not obeyed the gospel. It is the case that we believe in what is called the free offer of the gospel, but that does not mean that we are free to reject the offer. The title of Jesus, the one on who we call, is ‘Lord’, and in itself it stresses the authority of the message. In the gospel, there is a call to submission, to accept the terms of salvation laid out by the Lord.