Most Christians believe that a person must be saved to make it into the Kingdom of God. Is that true and only that?
Ro 10:9 because if thou shalt confess with thy mouth Jesus as Lord, and shalt believe in thy heart that God raised him from the dead, thou shalt be saved:
But what of those who do all that but don't know Jesus?
Mat 7:22 Many will say to me in that day, Lord, Lord, did we not prophesy by thy name, and by thy name cast out demons, and by thy name do many mighty works?
23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
Then we have people who still have a problem with sin. What about them.
Mat 5:19 Whosoever therefore shall break one of these least commandments, and shall teach men so, shall be called least in the kingdom of heaven: but whosoever shall do and teach them, he shall be called great in the kingdom of heaven.
The Bible is quite clear on nearly all areas you ask about. But, much is not clearly understood by the majority.
The kingdom of heaven is not in heaven, but it will be on earth ruled by rulers from heaven - thus, its name:
Ps 72: 7 In his days shall the righteous flourish, and abundance of peace till the moon be no more. 8 And he shall have dominion from sea to sea, and from the river unto the ends of the earth. 9 The dwellers in the desert shall bow before him, and his enemies shall lick the dust. (Please read all of Psalm 72 yourself)
Dan 7:
. 13 I saw in the night visions, and behold, there came with the clouds of heaven one like a son of man, and he came up even to the Ancient of days, and they brought him near before him. 14 And there was given him dominion, and glory, and a kingdom, that all peoples, nations, and languages should serve him: his dominion is an everlasting dominion, which shall not pass away, and his kingdom that which shall not be destroyed.
18 But the saints of the most high places shall receive the kingdom, and they shall possess the kingdom for ever, even to the ages of ages.
Time of transition
After Armageddon, there will be a Great Crowd who survived this; there shall also begin resurrections of the righteous and the unrighteous, but not the wicked.
the unrighteous: consists of all those who did not get judged wicked but who were neither Jews, nor Christian and didn't know these or didn't know about the Bible God and Christ. This includes all from the time of Adam to the present.
As we are told, even now in the world, there may be those who are permitted, though not Christian, to be given this chance to serve God
the righteous: consists of those who were Jews of ancient Israel, perhaps even some Jews from the present if they are lucky, and Christians who do what they can to live in Christ holy lives.
This happens during the 1000 years of rule by Christ. Once satan is let out of prison, everyone on earth gets tested and many fail. Those who fail will be destroyed forever together with satan and his angels.
All humans sin, even those who do their best to live holy lives in Christ. The difference between sin and being damned for sinning is 'the practice of deliberate serious sin.'