My take on 1st Corinthians 15:2 is quite different from yours.
and by which you are being saved, if you hold fast to the word I preached to you—unless you believed in vain. 1st Corinthians 15:2
Paul has set out to clear up some misunderstanding among the Christians in Corinth about resurrection. He began in the previous verse by asking them to remember the gospel as he first preached it to them when establishing the church in Corinth.
Paul will insist that the content of the gospel, and their belief in all of it, is the foundation for everything that is true. He has reminded them that they received the gospel as it was presented to them; they continue to have a right standing before God because of their faith in the truth of the gospel.
Now Paul adds that they are in the process of being saved by the gospel right now. When Paul and other New Testament writers talk about salvation, they often use the past, present, and future tenses. Those in Christ
have been saved, in the sense that our sins are forgiven and our place in eternity is secure. We
are being saved; God is active right now in sanctifying us to be like Christ. We
will be saved when the moment comes for us to stand before God in eternity, and we're freed from all sin.
There's a problem, however. Paul describes those who have received the gospel as "being saved" if they hold firmly to the truth as he preached it to them. The word "if" is easily interpreted as "since," because this statement is tied to them holding fast to the word preached to them. But the Corinthians have believed the gospel in vain if they did not and are not believing the gospel as he preached it to them. In other words, if anyone is believing a false version of the gospel of Jesus, that person should not think that they have any standing before God.
The part of the gospel Paul will emphasize in this chapter is faith in Christ's resurrection, as well as His death. Both must be believed to hold to the gospel as Paul preached it to them.
BibleRef
The same goes for 1 Thessalonians 4:15. It explains the rapture perfectly.
For this we declare to you by a word from the Lord, that we who are alive, who are left until the coming of the Lord, will not precede those who have fallen asleep. 1 Thessalonians 4:15
First Thessalonians 4:13–18 represents an important shift in the subject matter of Paul's letter. Realizing the Thessalonian believers needed further teaching about Jesus' return (the rapture) and specifically about what happens to Christians who die before Jesus returns, Paul turns to address these vital matters. He wants his readers to know that death should not cause them to grieve like unbelievers. There is a bright prospect of seeing departed believers again. When Jesus returns from heaven, He will bring deceased Christians with Him. At that time there will be a loud command, the archangel's voice will be heard, and God's trumpet will sound. Christians who have died will receive their resurrected bodies, and living Christians will be caught up to join them, to be with the Lord forever. Paul urged the Thessalonian believers to use what he wrote about the rapture to encourage one another.
BibleRef
I don't see anything about the rapture in 2nd Peter chapter 3. Here is Peter’s answer to those who deny or distort our faith and hope regarding the second coming of Christ...
The
second coming is described in Revelation 19:11-16. The important differences between the
rapture and
second coming are as follows: 1) At the
rapture, believers meet the Lord in the air (1 Thessalonians 4:17). At the
second coming, believers return with the Lord to the earth (Revelation 19:14).