In the Old Testament God promised a coming day when His fundamental relationship with His people would change.
I will sprinkle clean water on you, and you shall be clean from all your uncleannesses, and from all your idols I will cleanse you. And I will give you a new heart, and a new spirit I will put within you. And I will remove the heart of stone from your flesh and give you a heart of flesh. And I will put My Spirit within you, and cause you to walk in My statutes and be careful to obey My rules. (Ezek 36:25-27)This appears to be an excellent description of what occurs in one who repents and believes. Jesus called it "born again" (John 3:3) and said this new birth is "of the Spirit" (John 3:5-6). Considering our old self, this is expected and necessary.
The Bible describes those who receive Christ as "children of God" (John 1:12), specially called to be conformed to the image of Christ "in order that He might be the firstborn among many brothers." (Rom 8:28-29) We are part of the family of God.
Paul described believers as a "new creation" (Gal 6:15), something entirely new. "If anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come." (2 Cor 5:17) He speaks of "the renewal of the Holy Spirit (Titus 3:5). Peter says we become "partakers of the divine nature" (2 Peter 1:4).
The largest part of this whole thing is centered on the unique relationship with the Spirit. We are described as the temple of the Spirit (1 Cor 6:19), the place where He lives (Ezek 36:27; John 14:17; 2 Tim 1:14; Rom 8:11). Before this we were described as "in the flesh" and unable to please God (Rom 8:5-8); now we are indwelt by the Spirit (Rom 8:9-11). The Spirit living in the believer has multiple effects. Remember that God told Israel He would "cause you to walk in My statutes and be careful to obey My rules." So we are filled with the Spirit (Eph 5:18) wherein He leads us (Gal 5:18) in all truth (John 16:13; 1 Cor 2:12) and produces fruit in us (Eph 5:22-23). We are "caused" to "walk in My statutes" because we have the Spirit indwelling us, "both to will and to work for His good pleasure."
The Bible is quite clear that this "new creation", this "new birth", this "heart of flesh" produced by the indwelling of the Spirit cannot go unnoticed, so to speak. You can't become one with Christ and not change. James says that saving faith always produces works (James 2:14-26). Faith that produces no changed behavior is dead faith. But this new faith and the work of the Holy Spirit produces work. So Paul tells the Philippians, Therefore, my beloved, as you have always obeyed, so now, not only as in my presence but much more in my absence, work out your own salvation with fear and trembling, for it is God who works in you, both to will and to work for His good pleasure." (Phil 2:12-13) We work, work hard, work with fear and trembling, because God is at work in us giving us both the will and the power to do what pleases Him. And this work is one of the things for which we were saved. "We are His workmanship, created in Christ Jesus for good works, which God prepared beforehand, that we should walk in them." (Eph 2:10)
One aspect of this salvation for the saved is assurance. John wrote his entire first epistle with this aim: "I write these things to you who believe in the name of the Son of God that you may know that you have eternal life." (1 John 5:13) We can know. It isn't a question. It isn't elusive. Paul told the Philippians, "I am sure of this, that He who began a good work in you will bring it to completion at the day of Jesus Christ." (Phil 1:6) Jesus said, "I give them eternal life, and they will never perish, and no one will snatch them out of My hand. My Father, who has given them to Me, is greater than all, and no one is able to snatch them out of the Father's hand. I and the Father are one." (John 10:28-30) There is no ambiguity in this text. "No one" does not include anyone at all. So while we have the Holy Spirit at work in us causing us to work out our salvation from our end, we have the absolute confidence that God "is able to keep you from stumbling and to present you blameless before the presence of His glory with great joy ..." (Jude 1:24-25) Our confidence isn't in us, our faith, or our faithfulness; it is in Him. Consider this sequence of thoughts:
Everyone who believes that Jesus is the Christ has been born of God, and everyone who loves the Father loves whoever has been born of Him. (1 John 5:1)This leaves no room for error. Born of God -> overcomes -> no second death. No deviation. No question. Similar to Paul's chain. "Those whom He predestined He also called, and those whom He called He also justified, and those whom He justified He also glorified." (Rom 8:30) From our perspective, there is work to do. From God's perspective, the answer is complete.
For everyone who has been born of God overcomes the world. And this is the victory that has overcome the world -- our faith. Who is it that overcomes the world except the one who believes that Jesus is the Son of God? (1 John 5:4-5)
"The one who overcomes will not be hurt by the second death." (Rev 2:11)
There are lots of benefits to the believer. We are one with God. We have a new heart. We are part of God's family. We are indwelt by the Holy Spirit. We work to obey, but we work to obey because He works in us. We have complete confidence in His ability to carry us through. There is the absolute assurance that the day will come that we will cease from our labors and enter into His rest, united with Him. Saved for what? For His magnificent glory and our great benefit. The good news; the Gospel.