I got this question:

“Where does Jesus Himself say that he died for sinners once and for all, and where does Jesus say that inside all men have the nature of Adam, the Nature of a sinner. And what was Adam's Sin, Wasn't it Eve that was the Sinner??”

My Answer:

To understand the atonement of Christ and salvation, I suggest that you take a pink highlighter and mark all the passages in a Bible on that subject, as I have done, not just the “sayings of Jesus.” However, I will provide some quotes to you from the gospels and elsewhere.

Matthew 20:28 (Mark 10:45): “. . . the Son of Man came . . . to give His life a ransom for many.”
This refers to one event in history, the crucifixion.

In John 19:28: “It is finished.” This is one of the last things that the Lord Jesus said on the cross.

Acts 20:28: “. . . the Church of the Lord, which He purchased [past tense!] with His own blood.”
This is a reference to the cross.

Romans 5:8-19: But God commends his own love toward us, in that, while we were yet sinners, Christ died for us. 9 Much more then, being now justified by his blood, shall we be saved from the wrath of God through him. 10 For if, while we were enemies, we were reconciled to God through the death of his Son, much more, being reconciled, shall we be saved by his life; 11 and not only so, but we also rejoice in God through our Lord Jesus Christ, through whom we have now received the reconciliation. 12 Therefore, as through one man sin entered into the world, and death through sin; and so death passed unto all men, for that all sinned: — 13 for until the law sin was in the world; but sin is not imputed when there is no law. 14 Nevertheless death reigned from Adam until Moses, even over them that had not sinned after the likeness of Adam’s transgression, who is a figure of him that was to come. 15 But not as the trespass, so also is the free gift. For if by the trespass of the one the many died, much more did the grace of God, and the gift by the grace of the one man, Jesus Christ, abound unto the many. 16 And not as through one that sinned, so is the gift: for the judgment came of one unto condemnation, but the free gift came of many trespasses unto justification. 17 For if, by the trespass of the one, death reigned through the one; much more shall they that receive the abundance of grace and of the gift of righteousness reign in life through the one, even Jesus Christ. 18 So then as through one trespass the judgment came unto all men to condemnation; even so through one act of righteousness the free gift came unto all men to justification of life. 19 For as through the one man’s disobedience the many were made sinners, even so through the obedience of the one shall the many be made righteous.

Get that? Reconciliation is by the death of Christ ( single event). Through one act of righteousness [ = Christ’s substitutionary death on the cross] comes the free gift of justification. Through Christ’s obedience in going to the cross, men may become righteous. The crucifixion is a single act.

Hebrews 9:11-14 But Christ . . . through his own blood, entered in once for all into the Holy Place, having obtained eternal redemption. . . . 14 how much more shall the blood of Christ, who through the eternal Spirit offered himself without blemish unto God, cleanse your conscience from dead works to serve the living God?

Here is a reference to the cross, one historical act.

Heb 9:26-28: For Christ did not enter into a holy place made with hands, like in pattern to the true; but into heaven itself, now to appear before the face of God for us: 25 nor yet that he should offer himself often, as the high priest enters into the holy place year by year with blood not his own; 26 else must he often have suffered since the foundation of the world: but now once at the end of the ages hath he been manifested to put away sin by the sacrifice of himself. 27 And inasmuch as it is appointed unto men once to die, and after this cometh judgment; 28 so Christ also, having been once offered to bear the sins of many, shall appear a second time, apart from sin, to them that wait for him, unto salvation.”

Note the words: “nor yet that He should offer Himself often,” “now once . . . the sacrifice of Himself,” “offered once to bear the sins.”

1 Peter 3:18b “Christ also suffered for sins once, the righteous for the unrighteous, that He might bring us to God.”

“Where does Jesus say that inside all men have the nature of Adam, the Nature of a sinner.”

The Lord Jesus clearly believed all of the Bible which had been written during His ministry on earth. And He also believes all of the Bible today. He taught in John 3 that to enter the Kingdom of God a man must have a new birth. The Bible traces all men back to Adam, including Eve made from one of his ribs. Thus our physical natures manifested at birth go back to Adam. Genesis 5 records that Adam begat a son in his own likeness, after his image. Since scripture includes all men in Adam’s sin (see Romans 5) and since sin is a willful act of a person, this requires that not only the physical nature of man, but also the spiritual nature must be derived from Adam. Ephesians 2:3 calls all men by nature children of wrath.

“Wasn’t it Eve that was the sinner.”
Good question. Well, they both sinned. Eve was deceived by the serpent. But evidently Adam was not deceived and sinned deliberately.
The Bible does tell us that in sin did my mother conceive me. Our sin natures go back to conception and thus through both parents back to Adam. You could also read about “the old man,” in Romans 6. It is important to realize that we don’t just sin by choice, but we are born with a sinful nature. Another word sometimes used for our sinful nature (or Adamic nature) is “flesh,” as in Romans 7.