Romans 1:16:
16) "For I am not ashamed of the gospel of Christ: for it is the power of God unto salvatin to every one that believeth; to the Jew first, and also to the Greek (Gentile: every nation that is not Israel)."

 

I am a duly-appointed, sent, and fully qualified (by Jehovah God, my heavenly Father):

 

'Ambassador For Christ' (II Corinthians 5:19-21)

The most important words that I can say to anyone and everyone on planet Earth is this:

'Christ died for your sins.'

Even though Paul, an apostle of Christ, knew deep mysteries of the only God (Jehovah/Yahweh, by interpretation), this witness of Jesus Christ through the gospel of Christ, isolated this following passage to declare to all people, and all peoples, the most important message in the universe:

I Corinthians 2:2:
2) “For I am determined not to know anything among you, save Jesus Christ, and Him crucified.”

Why is that the most important message above all others?

Because it is the gospel of Jesus Christ for the salvation of the lost souls of all mankind, as God stated through Paul in this passage of scripture:

Romans 5:8:
8) “But God commeneth His love toward us, in that, while we were yet sinners, Christ died for us.”

Why was it necessary that Christ die for us? Aren't we alright just like we are? What was accomplished through Christ dying for all of us?

In the beginning, God created all natural things: the universe; the heaven and the earth. Then God made man in His own image, after His own likeness. And that meant that man was pure, and innocent of sin, for sin had not entered into the flesh of man after God made him.

God gave the man one commandment (law) in the garden of Eden:
Genesis 2:16-17:
16) “And the Lord God commanded the man, saying, 'Of every tree of the garden thou mayest freely eat:

17) “But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.”

God formed woman from the part of Adam's body which God took (which we commonly call 'Adam's rib'), and Adam called her name 'Eve'. (Genesis 1:26-28; 2:22; 3:20)

However, Satan (represented by a 'serpent', or a 'dragon' in the garden of Eden, refuted God's word in the commandment which God gave the man, Adam, and by delusion, deceived Eve, so that she took of the forbidden fruit, and ate of it, and then gave to her husband, and he did eat. And immediately they saw that they were naked, and became ashamed, and hid from God. (Genesis 3:1-8)

And that was the sin which Adam committed, for he had transgressed the law of God, 'for sin is the transgression of the law': (Romans 4:15; I John 3:4).

How did that affect everyone else that lived after Adam and Eve?

Romans 5:12:
12) “Wherefore, as by one man (Adam) sin entered into the world (by Adam's flesh that had transgressed the law of God which God gave to the man, which flesh we also inherited from the lineage of Adam: 'sinful flesh'), and death (came) by sin; so death passed upon all men, for that all have sinned:”
Hebrews 9:27-28a:

27) “And as it is appointed unto man once to die, and after that the judgment:
28a) “So Christ was once offered (as the only Sacrifice acceptable to God for the taking away of the sins of man) to bear the sins of many (whosoever believeth in Him: Jesus):”

John 3:16-18:

16) “For God so loved (every person in) the world, that He gave (for the Sacrifice for each and every person's sins which we have all committed) His only begotten Son (direct Descendant of God, the Holy Ghost: Matthew 1:18), that whosoever believeth in Him should not perish (as a hopeless sinner into eternity), but have everlasting life.
17) “For God sent not His Son into the world to condemn (any person in) the world, but that (each and every person in) the world through Him might be saved (from their past sins which we have all committed, and the eternal penalty of our unforgiven sins).
18) “He that believeth (in Christ) is not condemned (any longer); but he that believeth not is (still) condemned already, because he hath not believed in the name of the only begotten Son of God.”

So then, that is why this is the most important message to every person in the world, because when we come to the knowledge of good and evil, we then become guilty of all the sins we have committed up to that point in our lives, and we are then 'dead in trespasses and sins': Ephesians 2:1.

Paul, the apostle of Jesus Christ, wrote it this way:
Romans 7:9:

9) “For I was alive without the law once (innocent from sin, not knowing 'good and evil'): but when the commandment came (into my knowledge and understanding of sin), sin revived (sprang up into my knowledge, and in my memory), and I died (in trespasses and sins which I had committed).

So then, the most important thing in anyone's life on this earth is to know Jesus Christ as your Savior, from the sins of our past.

Even though God has freely given me a far greater understanding in the 'word of truth' (the Christian Bible) than I ever dreamed I could have, this message of your eternal salvation is the first and best thing in all of that blessed knowledge and understanding that I could give anyone.

And when one believes that gospel of salvation in Christ Jesus, I am overjoyed, because another soul has been 'plucked' from an eternal fate, and placed into the love of God, and into His family, for whom the universe was made; for God wants to build His family upon whom He will bestow the purest love man could ever know upon everyone that will believe in His Son Jesus Christ unto their own redemption, and share that new heaven and earth which He shall build for us, with Himself for evermore, with total peace, joy, and love unending.

Ref: Revelation 21:1-5.