Do those good people who die without knowledge of God go to hell, or do they go to heaven because they were good, unknowingly ignorant people?
Jesus answers this question:
Luke 12:47-48
47 And that servant who knew his master's will, and did not prepare himself or do according to his will, shall be beaten with many stripes.
48 But he who did not know, yet committed things deserving of stripes, shall be beaten with few..."
God judges us according to our knowledge. Those who did not know of the Gospel will receive "a few stripes." That is, their punishment will be less than those who knew the truth of the Gospel and refused it.
No one can claim complete ignorance of God. Sure, many may try to, but God's Word is clear that every human person has some sense of God's existence. Unfortunately, most humans suppress their innate knowledge of God in favor of serving themselves. Paul the apostle explains:
Romans 1:18-21
18 For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who suppress the truth in unrighteousness,
19 because what may be known of God is manifest in them, for God has shown it to them.
20 For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse,
21 because, although they knew God, they did not glorify Him as God, nor were thankful, but became futile in their thoughts, and their foolish hearts were darkened.
We all have some light, some basic understanding that God is, and God holds us accountable for what we do with that light. If we seek to know more about God, to increase our light, God promises to respond and to meet us in our search for Him.
Jeremiah 29:13
13 And you will seek Me and find Me, when you search for Me with all your heart.
This was God's promise to the Israelites and He makes a similar to one to us today:
James 4:8-10
8Draw near to God and He will draw near to you. Cleanse your hands, you sinners; and purify your hearts, you double-minded.
9 Lament and mourn and weep! Let your laughter be turned to mourning and your joy to gloom.
10 Humble yourselves in the sight of the Lord, and He will lift you up.
Many are the stories of God reaching into the lives of people far removed from the sound of the Gospel and bringing them to faith in Christ. When we search for God sincerely, He will move mountains if necessary to be found by us. There is, then, no person who will one day stand before God and be able to legitimately say, "I had no idea you even existed!"
If they go to hell - how could you possibly worship such a monster?
It is only those who are steeped in sin and perfectly comfortable with it that rise up in protest and call the holy One who will judge and punish that sin a "monster." So deeply entrenched in sin and darkness, such people cannot fathom God's hatred of something with which they so are so easy and from which they take so much pleasure. It never occurs to them that they are the one's whose attitude and response to sin is in error. They are too far into darkness to have such clarity about it. But God sees sin for what it really is and His wrathful judgment upon it will not be stayed even though those who love their sin think Him a monster.
All those who stand before God's judgment seat will receive precisely what they ought to receive. No person will ever stand before God good enough to satisfy His perfectly holy and righteous standard. That is why we all so desperately need Christ and his perfect righteousness to atone for our sin and cover us.
If they go to heaven - then aren't missionaries actually damning millions to hell worldwide?
Missionaries don't damn anyone to hell. People do that for themselves. See above. The Good News is that God has made a way for any who wish to escape His judgment to do so. No one has to go to hell. As C.S. Lewis noted, the door to hell is locked
from the inside.
Selah.