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You, us, we, cannot guarantee our eternal destiny. Only Christ can do that, and with Him it is a done deal once we become one of His disciples. You can make sure that this does not happen, however, by choosing to not become His disciple, which is your prerogative. All we can do is choose or not choose, Jesus guarantees the outcome.
In the UK today, as I visit a few small churches to share the Gospel of Jesus Christ, i have become convinced the single biggest issue facing the Church today is Knowing they are Born again.
Jesus is our Assurance: Our acceptance of Him as Saviour guarantees our eternal destination - though it is true we can do nothing towards it. He is Redeemer - He alone is the Propitiation fro our sins; It's Jesus - Jesus - Jesus.
We cannot guarantee our eternal destiny is (strictly speaking) true; But He can! and I can be sure of my relationship to him.
Absolutely looking forward to eternity in Glory! with Jesus - see you there?
Why is salvation by faith only? Because it is in believing that we can do nothing to save our selves and only the death and resurrection of Jesus can save sinners. When we first believed in Him His spirit joins our spirit and we are complete in Him, and we are sealed unto the day of the redemption of our spiritual bodies.
Now our hope is in Him who saved us by grace without works of righteousness which we have done. No one can make themselves good enough to go to heaven. It is by grace we are saved, not of works, lest any man should boast.
We can add nothing to Christ's finished work on the cross to save us guilty sinners. No amount of keeping Gods laws can save us. We are no longer under Gods laws because we are under grace and Jesus has paid for all our sin.
When we put our faith and trust in Him to save us He comes into our lives and begins to work in us to do the things he wants to do in us as He is the author and finisher of our faith.
Now we love Jesus for what He has done for us and allow Him to live out His life through us. We do good works because we are saved and not to save us. Good works can save no one. After we are saved we still need to rely on His perfect work to save us and not on our performance. Our salvation depends only on His performance which was perfect.
Now when God looks at us He sees the Holy Spirit that lives in us and declares us perfect before God. God can only accept perfection as He is perfect. But Jesus gave us His perfect righteousness in place of our unrighteousness the moment we trusted in His finished work on the cross as a free gift. Believers can no longer be brought in to condemnation because Jesus has paid for all our sin. John 3:16
After we have been saved by faith in His finished work on the cross, and we live in gross sin, the Lord will chasten us as a father chastens His children. But He will never condemn us. He only chastens us because He loves us. Once we become a child of His He will never leave us or forsake us. He loves us with and everlasting love that we cannot understand it is so wonderful. That is the Good news of salvation. We were dead in trespasses and sin until Jesus took away our sins and cast them as far as the east is from the west. Now we are a new creation born of God and not of man.