The Gospel is always preached to unbelievers. Why would believers need to hear
anew what they've already believed? Paul is reminding them again of what the Gospel entails. When Paul says, "our sins", he is talking to those who have already believed the same Gospel he has preached in the past. You would know this if you actually read what is written.
They had received it, believed it, and stood by it. "By which ye are saved". They are no longer unbelievers.
He reminds them again of what that Gospel was that he had preached to them when they were unbelievers.
1 Cor. 15:1 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;
I will quote the Apostle Paul again...same message, earlier date.
Acts 13:38 Be it known unto you therefore, men and brethren, that through this man is preached unto you the forgiveness of sins: 39 And by him all that believe are justified from all things, from which ye could not be justified by the law of Moses.
The Gospel is always preached to unbelievers. Believe in the Lord Jesus Christ, and you will be SAVED, and justified from all things. Unbelievers won't be justified unless they believe, nor will they be saved.