Pagan simply means unbeliever. Our Lord Jesus Christ invites all to be believers in Him and not unbelievers. In refering to "pagan", in the general theological sense, all we are saying is that something is not a profession of faith in Jesus Christ. Our Lord Jesus Christ is the God-man sent by God the Father for salvation for all who believe in Him. This is the gospel.
16 ¶ For I am not ashamed of the gospel. For it is the power of God unto salvation to every one that believeth: to the Jew first and to the Greek. (DRV) The "Greek" in this verse represents all peoples. The "Jew" in this verse means those Jews like the Apostles of Christ (not including Judas Iscariot) who believed in Christ and preached His gospel.