Is eternal security in Christ some fanciful concept or biblical truth?

 Why do some Christians believe a child of God can lose salvation when the promises of eternal life are all based on what Christ did at the cross?

As believers in Christ, we must understand our position in Christ. There is nothing we can, or ever will do to earn or attain Salvation. Not only did Christ pay it all, but planned it from the beginning, so to be HIS, then not HIS, is saying what he did was not sufficient. Salvation is solely based on what Christ did at the cross. Christ is the author and finisher of our faith. therefore, we are eternally secure in Christ because of our position in Christ. This new identity profoundly impacts our relationship with God and our families, as well as our perspective on the world. Because of our new identity in Christ, we now have the same relationship with God that Christ does—we are His children. God has taken us in as sons. We might address Him as "Abba! Father!"

We are both joint heirs and Christ's friend. And this bond is even stronger than what we have with our natural family. Rather than fearing God as a judge, we have the unique luxury of approaching Him as our Father. We can approach Him confidently and ask Him for what we need. We may seek His wisdom and counsel. We also submit to His authority and respond to Him with trusting obedience, understanding that obedience is an essential aspect of remaining close to Him.

• 2 Cor 5:17 - Therefore, if anyone is in Christ, he is a new creation; old things have passed away; behold, all things have become new.
  
• John 1:12 - But to all who believed him and accepted him, he gave the right to become children of God.
• 2 Corinthians 5:21 - For He made Him who knew no sin to be sin for us, that we might become the righteousness of God in Him.
  
• John 6:37 - All that the Father gives me will come to me, and whoever comes to me I will never cast out.
• Romans 8:30 - And those He predestined, He also called; those He called, He also justified
• Romans 8:29 - For those whom He foreknew He also predestined to be conformed to the image of His Son, in order that He might be the firstborn among many brothers.
• Ephesians 1:11 - In Him we have obtained an inheritance, having been predestined according to the purpose of Him who works all things according to the counsel of His will

Also, as believers in Christ, we tend to forget the purpose of the holy spirit. The holy spirit doesn’t dwell in our heart to look invisibly pretty:) but to ultimately bring glory to God, shaping us in the image of Christ. In order to do this the paraclete’s role is to guide us, convict us, heal us, warn us, transform us, etc., so when we fail, he draws us back to himself. No amount of preaching, pleading, or finger-pointing will result in sin-conviction unless the Holy Spirit is at work in the sinner's heart

It is the Spirit's responsibility to convict. And what is the most fundamental sin of which the entire universe is guilty? It is disbelief, according to Jesus. The Holy Spirit's convicting power is at work in the world "because they do not believe in [Christ]." When a person reacts to the Spirit's conviction and comes to trust in Christ, the other sins he committed are forgiven.

The world must also be convicted of righteousness, which the Holy Spirit also does. Despite the world's willful denial of the truth, we are all held to a righteous standard. And who is the righteous standard-bearer? Jesus identifies Himself as the standard, saying, "Because I go to the Father." There is only one Person who descended from heaven, lived a spotless life, and ascended back to heaven—the Son of Man, who lives to be our Intercessor. Christ demonstrates the righteousness that the world attempts to reject. Everything He said and did was a perfect reflection of God the Father. The Bible is quite clear that the Holy Spirit is active in our world.

In a conclusion, remember, the same God who saved you is the same God who will keep you. Once we are saved, we are always saved. If God said it, he will perform it!  Our salvation is most definitely eternally secure!

• John 1:12 - But to all who believed him and accepted him, he gave the right to become children of God.
• Romans 5:20 - Now the law came in to increase the trespass, but where sin increased, grace abounded all the more,
• Phil 1:6 - being confident of this very thing, that He who has begun a good work in you will complete it until the day of Jesus Christ;
• 2 Cor 5:21 - For He made Him who knew no sin to be sin for us, that we might become the righteousness of God in Him.
• 2 Cor 5:17 - Therefore, if anyone is in Christ, he is a new creation; old things have passed away; behold, all things have become new.
• Eph 1:13 - In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise,
• Eph 1:6 - having predestined us to adoption as sons by Jesus Christ to Himself, according to the good pleasure of His will,
• Titus - 3:5–7 - not by works of righteousness which we have done, but according to His mercy He saved us, through the washing of regeneration and renewing of the Holy Spirit, whom He poured out on us abundantly through Jesus Christ our Savior, that having been justified by His grace we should become heirs according to the hope of eternal life.
• Romans 8:38-39 - For I am convinced that neither death nor life, neither angels nor demons, neither the present nor the future, nor any powers, neither height nor depth, nor anything else in all creation, will be able to separate us from the love of God that is in Christ Jesus our Lord