How does Christ’s sacrifice cover sins after His death?

How can we help an unbeliever understand that Christ’s death is applicable for the sins after His death as well?

I once heard about a five-year-old child asking his mother: “Mom, Jesus died 2,000 years ago. How can He die for me, since I was born just five years ago?”

Often, people have doubts regarding the forgiveness of those who lived before and after Christ. Does His death still cover the sins of those who live centuries after the cross and resurrection?

Christ’s death is the most defining moment in human history, as it makes right all the things that went wrong regarding man’s existence on earth. The most innocent individual of all time died a wretched death as He bore the sins of mankind.

Christ’s death is the most defining moment in human history, as it makes right all the things that went wrong regarding man’s existence on earth

As we answer this question, we need to remember six peculiarities of Christ’s death:

1. What makes Jesus’s death valuable in the sight of God the Father is that He is fully God and fully man. Because of His divine nature, He is infinitely valuable, more than all people of all time put together. This is why His death is sufficient for people of all time.
2. His perfect humanity makes Him the perfect substitute for all time. He perfectly obeyed the Father, unlike everyone who lived before and after Him. His perfect record saved those in the Old Testament who looked forward to Him, and also us, who live after His death and look backward to Him — the Person upon whom the eternity of the world rests.
3. His death for the sins of the world was accepted by God. This was proved by His resurrection, which declared His sonship to the Father and vindicated Him (Romans 1:1-4). No one else before or after Him has demonstrated their victory over the perennial issue of death like Jesus did, because they never had any such credentials.
4. Jesus offered a single sacrifice for all time and sits at the right hand of the Father, having completed the righteous demands of God, unlike all of the Old Testament sacrifices, which the Jewish priests sacrificed daily (Hebrews 10:11-14). Those priests kept offering the same sacrifices year after year, because it was impossible for the blood of bulls and goats to take away sins (Hebrews 10:1-4). This means that His death accomplished the perfection that God looked for.
5. Jesus’s death and His victory over Satan and sin were prophesied in Genesis 3:15, when God said to the serpent that the seed of the woman would crush the serpent’s head, while the serpent would bruise the heel of the seed of the woman. God said it much earlier than the time we live in now — more than 6,000 years ago, to be precise. It was mentioned way before Christ came to this world to die. No one else was foretold with such importance in history. 
6. Jesus, who was the one foreknown to be our Saviour before the foundation of the world, was made manifest in the last times for the sake of everyone (1 Peter 1:20), so that everyone in this world may look to Him and be saved.

This perfect, infinitely valuable Son of God is the reason why we receive eternal forgiveness, even though we live many centuries after His life and death on earth.

Conclusion

Every person who lived before Christ was saved by looking forward to the cross, which was the atonement God provided for sins. Everyone who lives after Jesus must look at the Christ who was crucified and obtain eternal life through belief in the crucified Messiah. The issue of how it can be valid after His death is not as significant as the fact that Jesus’s death covers the sins of all mankind, both before and after His time on earth.