There are different deaths. Death means a separation. Physical death comes when the spirit becomes separated from the body. Spiritual death is when one’s spirit is separated form Christ.
Paul, in his writing to the Galatians, explains it like this:
I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me. (Galatians 2:20 KJV)
Paul, himself, was a persecutor of the young church established by the blood of Christ and initiated by the apostles on the first Pentecost after Jesus’ death. The quote above was made by him after Jesus appeared to him in a vision on the road to Damascus, and after his baptism by Ananais. (Acts 9:1-18 KJV)
To explain his quote in modern day speech, he is simply saying that he died to his sins and “crucified” the old sinful spirit and now he lives with the spirit of Christ dwelling in him. If we are dead to sin, then we are alive in Christ. Paul also tells the church in Rome, this:
Likewise reckon ye also yourselves to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord. (Romans 6:11 KJV)
Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life. (Romans 6:4 KJV)
The above discussion explains the “new birth” and gives the usage of “born again” credence even though some use it in a sarcastic manner.
Those who are unwilling to die in the manner and process above have no part in the salvation by the blood of Jesus Christ. And, that is not just opinion, that is scripture. We are plainly told that all spiritual blessings are in Christ. Which excludes any being outside of Christ. (Read Ephesians 3:7)
Now, that is the first death. The second death comes when the body and spirit are separated. The body returns to the dust of the earth while the spirit returns to God who gave it. (Ecclesiastes 12:6-7)
Not only are we obligated to die the first death to be in Christ’s body, but a Christian must be strong enough to stand firm in the faith and, as the many martyrs in the early church, be willing to die physically rather than to renounce the name of Jesus Christ.
Fear none of those things which thou shalt suffer: behold, the devil shall cast some of you into prison, that ye may be tried; and ye shall have tribulation ten days: be thou faithful unto death, and I will give thee a crown of life. (Revelation 2:10 KJV)
This admonition was given to the early church to strengthen them during their exceedingly severe persecutions ahead. And the term, be faithful unto death, not only means be faithful until you die, but also means even unto death, or the giving your life if necessary to remain faithful. Many were tortured, burned at the stake, stoned to death, had their eyes gouged out and more, before they would renounce Christ. They were faithful unto death. Most Christians are fortunate enough today that this does not beset them. Yet, we must be prepared ahead and have the mindset that we would die as these martyrs did, in order to keep the faith. God knows the hearts of man.