Phil 1:29 - (AMPC) For you have been granted [the privilege] for Christ’s sake not only to believe in (adhere to, rely on, and trust in) Him, but also to suffer in His behalf. Nothing there about suffering "in addition to believing the gospel" as being the cause of salvation. You could never suffer enough to make up for your sins and earn your salvation.And believing the gospel is not the only requirement to enter the holy city. (See Philippians 1:29).
*Romans 1:16 - For I am not ashamed of the gospel of Christ, for it is the power of God to salvation for everyone who believes, for the Jew first and also for the Greek. 1 Corinthians 1:21 - For since, in the wisdom of God, the world through wisdom did not know God, it pleased God through the foolishness of the message preached to save those who believe.