As far as "once saved always saved", Baptist will say if there is no change then you probably never were saved; even if you 'went through the motions' so in that respect you can lose a false salvation. Only God can judge.
I have often heard the Baptist argument that one who appears to have lost his salvation was never saved to begin with. I have always considered that to be a “cop out” at best, and a clear violation of scripture at worst.
I agree that there are those who go through the motions without ever really having been saved. God knows who they are; I don’t. But to lump everyone into that boat is presuming to judge, when only God has that prerogative. I do understand that that argument is necessary because of Calvin’s other doctrines, especially the idea that man cannot choose on his own to be good and the idea that God only saves those he chooses to save and rejects those he selects for condemnation. I also understand that some man-on-the-street Baptists (as opposed to theologians) reject parts of Calvinist doctrine while keeping the conclusion that results from the parts they reject.
One of the strongest arguments, though, is that the scripture says that some who have fallen away were actually and unquestionably saved before they turned back to their old ways. “For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.” (Hebrews 6:4-6) The writer here describes one who has been saved. No Baptist I know will say that one can have the Holy Spirit without salvation. So the person who wrote Hebrews clearly thinks that one can be saved and then fall away, instead of it only being an “apparent” salvation.
If God says someone can be saved and then fall away, I don’t want to presume to judge that everyone that appears to fall away was never saved to start with. As you say, “Only God can judge.” Even in the parable of the soils (Matthew 13) Jesus says it is possible for some to be saved and then be choked out or lose their root.
In reference to Hebrews 6, if you read the whole chapter, you will notice in verse 9 and 10 that God is "not unfair". Notice the use of the double negative. I tell my child, " I will never not love you". In Hebrews 6 the message is one of mature faith. Because God's love is perfect. If you read verse 7, it relates how yes your life can be nothing, but still your salvation is secure. God will not abandon His children.
Have you ever talked with someone who once was saved but has lost their faith? That person is MISERABLE. Their life is miserable. No matter what their pretense, everything they grow is weeds. And who can save them who are already saved but living separate from God's light? Each of us has a responsibility to be eager in our faith.
The Hebrews says nothing about going back to your "old ways". In contrast, it talks about losing your faith. People lose their faith for various reasons and then redefine their walk, mature and move forward.
Certainly you are right that the message of Hebrews 6 is mature faith. That is, it is an encouragement toward mature faith. And certainly God is not unfair, and will not abandon his children. That does not imply that God’s children may not abandon him.
You mention verse 7, but omit the rest of the sentence in verse 8. Both are talking about people who have believed. While verse 7 is about how God blesses those who follow him, verse 8 contrasts that with those who, to use the phrase in verse 6, “fall away.”
And that brings me back to the writer’s argument in verses 4-6. I agree wholeheartedly with you that one may lose his faith and redefine their walk. I suspect that happens much more often than that one loses his salvation. But the writer here is not talking about “losing faith” but about “falling away.” This literally means to fall beside the path, to leave the path entirely. These people have not just lost faith, but they are actively denying that what faith they once had was of any value. They are like the Galatians who quit relying on Jesus for salvation, and instead said they could be saved by keeping a law perfectly. They are telling God, “I don’t want the sacrifice you made because it was unnecessary. I can do it on my own.”
The alternative, that you appear to be proposing, is that one can crucify Jesus and shame him without guilt. Talk about unjust! God can demand that his only-begotten son die on a cross as a sacrifice for sin, and then tell people that it is acceptable to reject him without consequence. You might as well propose that God saves everyone, regardless of faith. Of course, that would mean that Jesus did die on the cross for no purpose.
Regardless of all the above, whether everything I just said is right or wrong, it doesn’t change the original argument. Those verses still show the falsity of the idea that one who seemed to be saved and yet denies Christ must never have been saved in the first place.
Lest it sound like I believe that it is easy to lose one’s salvation, please see "Once Saved, One Sin" for my article about how some of us misrepresent our doctrine.