Faithful souls are primarily those individual believers who have faith in and are faithful to Jesus Christ.

But God, when He judges people who have not known Christ, still has ways of testing people to prove them faithful to the Good they know, as it has been revealed to them in their lives. God will save everyone who would truly enjoy Heaven. We may one day be surprised to meet in Heaven some of the people with whom the Holy Spirit has had fellowship.

You are absolutely right, Ron. Those who had the possibility of learning greater truth through greater light will be held responsible for what they could have learned but refused (EGW). All those from ages past that lived up to the light they were shown will be in Heaven even if they never heard the name 'Jesus Christ'. All those that refused to learn the truth when given the opportunity will be lost, even if they 'served the Lord' their entire lives. These people will be very upset with God for doing this to them, and during the Great Tribulation these people (who were once in the Kingdom and are now cast out) will know that they are lost forever and will become the most fearsome enemies of the righteous who are in and who join the Kingdom of Heaven.

And the foolish said to the wise, ‘Give us some of your oil, for our lamps are going out.’ But the wise answered, saying, ‘Since there will not be enough for us and for you, go rather to the dealers and buy for yourselves.’  And while they were going to buy, the bridegroom came, and those who were ready went in with him to the marriage feast, and the door was shut.  Afterward the other virgins came also, saying, ‘Lord, lord, open to us.’ But he answered, ‘Truly, I say to you, I do not know you.’ Matt.25:8-12.

But if that wicked servant says to himself, ‘My master is delayed,’ and begins to beat his fellow servants and eats and drinks with drunkards, the master of that servant will come on a day when he does not expect him and at an hour he does not know and will cut him in pieces and put him with the hypocrites. In that place there will be weeping and gnashing of teeth. Matt.25:48-51.