Tenses matter. Paul told the Corinthians they had received the gospel (1 Cor 15:1) which saved them–past tense. But then he said the gospel is also the message “by which you are being saved” (1 Cor 15:2)–present tense.
Put another way, sinners never exhaust their need for the gospel. We need it to become Christians, and we need it to live as Christians! The gospel is the message for both justification and sanctification.
Christians must hold to the gospel which saved them, for its work in their lives continues by the power of God. The Christian life should be gospel-centered because it is driven by God’s grace from beginning to end.
Paul told the Corinthians they would hold to the gospel “unless you believed in vain.” This means growing in the gospel is evidence of salvation. On the other hand, not holding to the gospel is evidence of unbelief. Are you holding to the gospel today?
The gospel is for Christians–which sounds like a good book. :)