The simplest answer to the question above... because we are all sinners (Romans 3:23 For all have sinned, and come short of the glory of God). And because we have all sinned, we are condemned men & women walking the earth. But because we were created by a merciful God, full of patience & grace, who gave His Son as a ransom to pay the full price of our sin (John 3:16 For God so loved the world, that he gave his only begotten Son) we can be saved.

Without this powerful gift of God, the Father, we would all face eternal damnation. But through the unselfish act of Jesus, the Son, we can live forever for all of eternity. But we must do our part to receive this free gift in order to attain eternal salvation.

(1 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;  By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.  For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;  And that he was buried, and that he rose again the third day according to the scripture).

This is the gospel of faith & faith alone in order to attain salvation. This is what every Christian's salvation rests on. It is the foundation of Christianity & the gospel of Christ for the Gentile & for any Jew who might believe. For if we believe in the death, burial & resurrection of Our Lord & Savior Jesus Christ, that He is the Son of God, & we place our hope in what he did on Calvary's Cross & we trust that His death was sufficient to cover our sins completely, then we shall be saved. This is true salvation & by you simply believing & never waivering, you shall see eternity when you leave this life. But more importantly, you will spend eternity with Christ Himself. 

So we learned what true salvation is, how to attain it & we will find out what it entails fully, but first, lets see how so many get it wrong.

First, you have those who try to add to what Christ did on the cross, which basically says that Christ didn't do enough. They add by trying to do things in order to get salvation, like going to church, tithing, not cursing, etc. And while all that is good to do & should be how all Christians behave...not going to church, not tithing & cursing like a sailor, etc., 'will not' keep you from being saved if you believe in Him crucified. This is the true meaning of being saved by faith & faith alone. In short, it's not what you do or don't do, it's what you believe & who you believe in, that will save you.

Second, you have those who just plainly get the gospel wrong. If you are under another gospel other than what Paul the Apostle preached, your salvation is not guaranteed. Paul himself said, (Galatians 1:9 If any man preach any other gospel unto you than that ye have received, let him be accursed). You have to get the gospel right in order to get salvation right.

Third, you have those who are denominationlist who believe that their denomination is the correct one & everyone else is wrong. They are all wrong & here's why. Paul the Apostle taught one gospel, not two, not several, but one. His writings on the gospel that the Holy Spirit Himself revealed to him can be found in the books of Roman thru Philemon otherwise known as the Pauline epistles. This is where the true Christian gospel can be found & what every Christian should know & be reading in order to get salvation correct.

Finally, you have those who teach that the entire Bible is for the modern Christian living in the church age also known as the grace period. This is absolutely wrong & here is why.

The Old Testament (Genesis-Malachi) is mostly historical books showing how Israel denied God from the beginning & how God was patient but the Jewish people were stubborn & set in their way, always running to other gods & other beliefs.

Then you have the four gospels (Matthew-John) which are transitional books showing a new gospel coming into effect that will be based on faith in God's Son, Jesus. These books are transitional because they show the Old Testament is ending & the New Testament is beginning, but the New Testament can't be established until the death, burial & resurrection of Christ is completed. This isn't accomplished until the end of these 4 books.

Next, you have the book of Acts (chapters 1-8), which is a historical book of the 12 apostles & how they struggled to bring in the Jews to believe in the Christ. Acts also showed the signs, miracles & wonders given to them by the Holy Spirit so that the Jewish people might believe, but even that didn't bring in the house of Israel to Christ or to God. Acts 9-28 is a historical accounting of Saul of Tarsus, becoming Paul the Apostle, the minister to the Gentiles & the Apostle to the Gentiles.

After Acts, comes the gospel for today's Christian (Romans-Philemon) also known as the Pauline epistles. Paul was the author of these books which make up 2/3rds of the New Testament. In these books can be found everything pertaining to the born again Christian, what a Christian walk looks like & what it means to be an Ambassador of Christ.

After which, comes Hebrews-Jude which is a futuristic instructional 'how-to' for Jews who are to live & will live during the 7 year Tribulation period & the Millenial reign of Christ on earth.

Finally, the book of Revelation, a look at heaven & God's thone, the Church & its good & bad points, the rapture & also a futuristic look of man after the rapture, in the Tribulation period, during the Millenial reign & going into eternal life. It also depicts the earth done away with,  heaven on earth & the renewing of the earth with Christ as the ruler & King.

So, No-not all of the Bible pertains to the Christian & especially to a Christians salvation. You can't apply what was meant for the Jewish people to the born again Christian & vice versa, For example, as a Christian, I can't live trying to be obedient to the law of Moses, I have to live in faith & faith alone. What I can do however, is learn from what the Old Testament showed about man's struggle to be obedient & I can come to realize that I'm going to fail at times but I am saved no matter what, as long as I believe in Him crucified. So, while I can learn from all of the Bible, as a Christian, I also have to reject what goes against the gospel I am under in order to keep my salvation intact.