In answer to a request for this information, I will endeavor to answer as simply as I can....
To understand the "salvation message" one first has to understand what salvation is. What are we getting saved from, exactly?
In reading the scriptures it is apparent that Adam lost something that only Jesus could get back for us. But why did it need him to sacrifice his life?
The scriptures say that Christ's sacrifice is a "ransom" which is the price paid for the release of a captive.
Jesus said...
"Just as the Son of man came, not to be ministered to, but to minister and to give his life as a ransom in exchange for many.”
What Adam did was sell his children into slavery due to his disobedience. They then had to pay for what he did. This was slavery to sin and death....not something that we can release ourselves from. But why did Jesus have to be born as a human child and offer up his life? Because the price required under the law of God was an
"eye for an eye, tooth for a tooth, life for a life."
Adam's sin caused alienation from God requiring a mediator to act as a go-between for us with God....because of our sinful condition, we cannot come before God directly. As mediator Jesus conveyed God's instructions to mankind behind the scenes for thousands of years before his mission to earth to offer his life for us. The "crime" committed by Adam carried the death penalty.....Adam died, but as a result, that penalty was inherited genetically by all of Adam's children. (Romans 5:12)
Since a perfect life was lost, only a perfect life could redeem mankind. None among the human race were now perfect, tarnished by the defect of sin. Jesus volunteered to carry out the mission and fulfill the law that justice required. It was the ultimate act of love. (John 15:13)
His life would pay for our release. But why is salvation necessary? Because humans on earth have free will and many of them do not want Christ's ransom to apply to them. By their choices and lifestyle they show God that they don't want to know him.
The apostle Paul wrote that....
"it is righteous on God’s part to repay tribulation to those who make tribulation for you. 7 But you who suffer tribulation will be given relief along with us at the revelation of the Lord Jesus from heaven with his powerful angels 8 in a flaming fire, as he brings vengeance on those who do not know God and those who do not obey the good news about our Lord Jesus. 9 These very ones will undergo the judicial punishment of everlasting destruction from before the Lord and from the glory of his strength, 10 at the time when he comes to be glorified in connection with his holy ones and to be regarded in that day with wonder among all those who exercised faith, because the witness we gave met with faith among you." (2 Thessalonians 1:6-10 NWT)
At the time of the judgment, only those who have proven worthy of the ransom will be saved from the coming wrath of God which will be expressed upon all those who heard the Christian message but rejected it.
Jesus used the days of Noah as a symbol of things to come....
"
For as were the days of Noah, so will be the coming of the Son of Man. 38 For as in those days before the flood they were eating and drinking, marrying and giving in marriage, until the day when Noah entered the ark, 39 and they were unaware until the flood came and swept them all away, so will be the coming of the Son of Man." (Matthew 24:37-39 ESV)
Just as surely as God brought a flood of punishment upon a world of ungodly people, so he will again. Only this time it will not be by water.