The gospel calls men to believe in Jesus Christ. The gospel condemns any other way unto salvation. A man is saved only by believing on Christ, who is proclaimed in the gospel. Man cannot save himself. Man cannot even cooperate in his own salvation.
Salvation is apart from any works that a man can do.
A man is saved only by believing in Jesus. Because salvation is only by believing the gospel, the faithful preacher of the gospel commands men to repent and to believe. This command to believe is an essential part of the gospel. A man is saved, therefore, by believing in Jesus Christ who is proclaimed in the gospel. In no other way can a man be saved.