This is difficult to understand. But we have to look at what God says.
Gods loves all people. The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance<. He desires that none should perish, But the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up (2 Peter 3:9-10).
Even in our day of sophisticated communications, many live and die without hearing the saving gospel of Christ. Some reason that a loving God will save those who have no chance to receive or reject the gospel. Christ said, however, Go ye into all the world, and preach the gospel to every creature. He that believeth and is baptized shall be saved; but he that believeth not shall be damned (Mark 16:15-16). He made no distinction between those who openly reject His gospel, and those who have not heard.
Paul warned that Jesus and His angels shall be revealed, In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ: Who shall be punished with everlasting destruction from the presence of the Lord, and from the glory of his power (2 Thessalonians 1:8-9). Vengeance is on them that know not God and that obey not the gospel.
Paul asserted that because of the evidence of creation men should know God and are thus without excuse. However, Professing themselves to be wise, they became fools, and changed the glory of the uncorruptible God into an image made like to corruptible man, and to birds, and fourfooted beasts, and creeping things Who changed the truth of God into a lie, and worshipped and served the creature more than the Creator. Because the heathen world rejected God and worshipped the creature rather than the Creator, God gave them up unto vile affections (Romans 1:20-26). If God will save the heathen in their sins, rather than from their sins, then Jesus died in vain. God would save ALL in their sins. Peter said, however, there is none other name under heaven given among men, whereby we must be saved (Acts 4:12). If the heathen are not really lost, why did Christ command, Go ye into all the world, and preach the gospel to every creature (Mark 16:15)?
Since all must obey the gospel, each who hears and understands should obey the gospel immediately, and work to share the gospel with the world. Paul warned, the times of this ignorance God winked at; but now commandeth all men every where to repent: Because he hath appointed a day, in the which he will judge the world in righteousness by that man whom he hath ordained; whereof he hath given assurance unto all men, in that he hath raised him from the dead (Acts 17:30-31). David warned, The wicked shall be turned into hell, and all the nations that forget God (Psalm 9:17). Our lost world needs the gospel of Christ.
As for babies and children:
Ezekiel 18:20 says, The soul that sinneth, it shall die. The son shall not bear the iniquity of the father, neither shall the father bear the iniquity of the son: the righteousness of the righteous shall be upon him, and the wickedness of the wicked shall be upon him. This one verse proves we did not inherit Adam's sin.
2 Corinthians 5:10 says, For we must all appear before the judgment seat of Christ; that every one may receive the things done in his body, according to that he hath done, whether it be good or bad.
Some denominations believe a baby is born into sin, such is not true. A child begins life in this world in purity and innocence. When children are small, they have no knowledge of good or evil (Deuteronomy 1:39). A child must grow up in order to know to refuse evil and chose good (Isaiah 7:15-16). This is why Jesus said the kingdom of heaven (another description of the church as I discussed in another post) is composed of people who are similar to little children (Mark 10:14). We have to become like children to enter the kingdom (Matthew 18:3). To enter the church, we must be cleansed of our sins, just as little children are without sin.
People are punished for their own sins (Colossians 3:25). God does not chose who is punished for sin or rewarded for righteous by individual. Rather, the actions of each individual determines whether they will be punished or rewarded. God set the standards. Our actions are measured against this standard. The decision is on man's part. God remains an impartial judge.