This text declares exactly what one must believe to receive salvation.
I Corinthians 15:1-4 -- Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;
2 By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
3 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
4 And that he was buried, and that he rose again the third day according to the scriptures:
The "if" is not conditional and could be translated " seeing that..."
Paul also said, "I did not come to baptize but to preach the gospel." ( I Corinthians 1:17).
The Gospel of the Circumcision requires repentance and water baptism, but the Gospel of the Uncircumcision ( I e. Romans) requires only belief.
I believe this verse sums it up for Christians today...
Ephesians 1:13b, 14 -- in whom also after that ye believed, ye were sealed with that holy Spirit of promise,
14 Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory.
The exact moment one believes, he or she is sealed with The Holy Spirit. Nothing can add to that or take away from it, especially works if the flesh.