… the gospel of God which he promised beforehand through his prophets in the holy Scriptures... (Romans 1:2)
There is much that could be said about this verse. For example, the gospel originates with God, by which Paul means the Father. The Father is the origin of good news. Not only that but he made it known through promises long before Jesus came. His people could anticipate its coming if they believed God. The method of delivery of the promise was through prophets, which will include Moses (Deut 18:15), who all wrote down what they received to form “the holy Scriptures”.
Interesting implication: the gospel did not begin with the advent of Jesus. It was present in the pre-Jesus history. In fact, if one looks closely at the story one sees it goes back to the Garden (Genesis 3:15). And one could argue it went back before that. How else could it be that “he chose us [i.e. Christians] in him [i.e. Christ] before the foundation of the world” (Ephesians 1:4)?
When we consider the uniqueness of Christ, and therefore the absolute necessity of eternal salvation only in and through him, the validation of that claim does not rest only on a couple of proof texts such as John 14:6 or Acts 4:12, though those are good places to rest. It is more pervasive than that. Those verses are the summary of the richer truth that the whole of human history has been heading in one direction, governed by a sovereign, loving Father, who has left signposts along the way pointing to the coming of his Son, who would do the work of saving his people.
There are not many ways of salvation. There is only one. There always has been only one. As long as history continues, there will ever be only one – the gospel of God.