The dead in Christ referred to by Apostle Paul on that verse are members of the Church founded by Christ. They are said to be in Christ because they are members of the body of Christ, or the Church of Christ (Romans 12:4-5, Colossians 1:18, Romans 16:16)

Take note, that those who died in Christ will rise first. After this, what will happen to them? According to Apostle John they are blessed and holy and that the second death has no power over them.

6 Blessed and holy is he who has part in the first resurrection. Over such the second death has no power, but they shall be priests of God and of Christ, and shall reign with Him a thousand years. (Revelations 20:6, NKJV)

What is Second Death, anyway? Apostle John said that the Second death is the lake of fire.

14Then death and Hades were thrown into the lake of fire. The lake of fire is the second death. (Revelations 20:14, NIV)

In other words, members of the true Church of Christ although experience death (the first death) will never experience the second death which is the lake of fire.

Why is it that members of the Church of Christ will never experience death? It is because Christ will save His Church.

23For the husband is the head of the wife as Christ is the head of the church, his body, of which he is the Savior. (Ephesians 5:23, NIV)

If Christ is the savior of His Church, does it mean that Christ will save His Church alone? Yes. Christ will save His Church alone, because according to Apostle Paul, outside the Church is God's judgment.

13God will judge those outside. "Expel the wicked man from among you." (I Corinthians 5:13, ibid)