Romans 1-2 answers this question fairly succinctly. I recommend that you read over these verses very carefully...
" 18For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who hold the truth in unrighteousness;
19Because that which may be known of God is manifest in them; for God hath shewed it unto them.
20For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse:
21Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened...
25Who changed the truth of God into a lie, and worshipped and served the creature more than the Creator, who is blessed for ever. Amen.
..32Who knowing the judgment of God, that they which commit such things are worthy of death, not only do the same, but have pleasure in them that do them.
1Therefore thou art inexcusable, O man, whosoever thou art that judgest: for wherein thou judgest another, thou condemnest thyself; for thou that judgest doest the same things.
2But we are sure that the judgment of God is according to truth against them which commit such things.
3And thinkest thou this, O man, that judgest them which do such things, and doest the same, that thou shalt escape the judgment of God?
9Tribulation and anguish, upon every soul of man that doeth evil, of the Jew first, and also of the Gentile;
10But glory, honour, and peace, to every man that worketh good, to the Jew first, and also to the Gentile:
11For there is no respect of persons with God.
12For as many as have sinned without law shall also perish without law: and as many as have sinned in the law shall be judged by the law;
14For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves:
15Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another;)
16In the day when God shall judge the secrets of men by Jesus Christ according to my gospel.
God made the conscience to "bear witness" of sin, to warn about what He disapproves of so that we can please Him with all our thoughts and actions. The bible says that after death every one of us shall give account of himself to God. [Rom 14:12] God will judge all men by His commandments to not lie, lust, and hate. He will also see if they obeyed His GREATEST COMMANDMENT, to love God with all of your heart, mind, soul, and strength. [Matt 23] Of course you have failed because all have sinned and fall short of the glory of God. [Rom 3:23]
God "has appointed a day when He shall judge the world in righteousness by that Man whom He has chosen." [Acts 17:31] The Man spoken of in this verse is Jesus Christ, the Son of God, who lived sinlessly on the earth. At the judgment all men will be compared to Him and anyone found less righteous than Jesus will be damned to hell forever, because they which commit such things are worthy of death. [Rom 1:32] and The wages of sin is death. [Rom 6:23]
This is fair. God is so pure and just that it is unthinkable for Him to accept sinners who hate obeying Him. In fact, God is angry with the wicked every day. [Psm 7] All men have rebelled against their conscience, so the wrath of God is revealed from heaven against all ungodliness and sinfulness of men. [Rom 1]
HOWEVER... John the Baptist and King David's child are at least two instances of babies and infants that were apparently regenerated at a very young age. This means God enlightened them to their need and softened them to receive the new nature. But, lest we question if some aboriginal has been born again in the boonies, we know that all who have the Spirit proclaim that Jesus is the Christ. If they deny Him before men, they were not saved. Anyways, the preaching of the word seems to be His only approved method for cognizant individuals outside of Mr. J. Baptist.
Hope this helps!!