For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes, to the Jew first and also to the Greek.
Romans 1:16 ESV

The gospel is the power of God to save all who believe, and not just the Jews to whom the promise of the gospel was first given. It is his power because there is power in his word. If he says something is true, then it is true. If he says that you will be saved, then you will be. "Everyone who believes" means that you must believe that God has come to set up his Kingdom and wants you to be restored to relationship with him in that Kingdom.