If you believe you can lose your salvation through sin, there's a couple of problems:
No. You have things backwards. There are problems if you don't believe the Bible's plain teaching that you can lose salvation by sin; For the Bible repeatedly teaches that sin can separate you from God.
"But your iniquities have separated between you and your God, and your sins have hid his face from you, that he will not hear." (Isaiah 59:2).
"Now we know that God hears not sinners: but if any man be a worshipper of God, and does his will, him he hears." (John 9:31).
“But whoso committeth adultery with a woman lacketh understanding: he that doeth it destroyeth his own soul.” (Proverbs 6:32).
“22 On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’
23 And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’” (Matthew 7:22-23 ESV).
“26 And every one that heareth these sayings of mine, and doeth them not, shall be likened unto a foolish man, which built his house upon the sand:
27 And the rain descended, and the floods came, and the winds blew, and beat upon that house; and it fell: and great was the fall of it.”
(Matthew 7:26-27).
“15 Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravening wolves.
16 Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles?
17 Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit.
18 A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.
19 Every tree that bringeth not forth good fruit is hewn down, and cast into the fire.
20 Wherefore by their fruits ye shall know them.” (Matthew 7:15-20).
1. What works cause you to lose your salvation?
Works of evil or grievous sin.
Grievous sin is another name for a "sin unto death"
(Note: To check out the reference to the "sin unto death," see: 1 John 5:16).
(i.e. death = spiritual death or the second death) (Note: The second death is destruction in the Lake of Fire - Revelation 21:8).
"And the LORD said, Because the cry of Sodom and Gomorrah is great, and because their sin is very grievous;" (Genesis 18:20).
Examples of Grievous Sin:
#1. Not loving God, and not loving your neighbor (For loving God and loving your neighbor is a part of eternal life; See: Luke 10:25-28 cf. Matthew 19:17-19; Not loving Jesus (God) means one is accursed, see: 1 Corinthians 16:22; As for not loving your neighbor, see the Parable of the Good Samaritan (Luke 10:29-37), and then see number #5 below).
#2. Looking at a woman in lust = danger of being cast bodily into hell fire (Matthew 5:28-30).
#3. Not forgiving = not being forgiven by the Father (Matthew 6:15).
#4. One can be condemned by their words (Matthew 12:37).
#5. Not helping the poor or the unfortunate = Going away into everlasting punishment (or everlasting fire) (Matthew 25:31-46).
#6. No man who puts his hand to the plow (i.e. one who spreads the gospel and teachings to lead men of God into holiness by His Word) and looks back (turns away from doing so) is fit for the Kingdom of God (Luke 9:62) (Note: See the KJV rendering on this verse).
#7. 1 John 3:15 says, "Whoever hates his brother is a murderer: and you know that no murderer has eternal life abiding in him."
#8. Galatians 5:19-21 says, "Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness, Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God." Meaning, those who do these kinds of sins will not inherit (enter) God's kingdom (i.e. they will not be saved). For entering God's kingdom is associated with salvation in Matthew 25:34 (Note: Paul is mentioning the violation of the Moral Law. The Moral Law is the same equivalent as loving your neighbor; See Romans 13:8-10).
#9. But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death. (Revelation 21:8).
You said:
How many do you have to commit?
It only takes one grievous unrepentant sin to be condemned by God. Adam and Eve died spiritually by their one sin. Ananais and Sapphira were killed when they lied to the Holy Ghost, and a great fear came upon the church, and all who heard about it. It is not normal for a church to be in fear over saved believers dying. They would feel remorse or sadness, but not fear if they knew they were safely in the arms of Jesus. The point here is why would anyone want to test God and find out? God warns us of the consequences of not obeying certain commands, and we should work out our salvation with fear and trembling. But today, there is no fear in the church.
You said:
What is considered really bad, bad enough to cost you your soul, and what is considered just a weakness of human nature?
Jesus recognizes that believers can struggle with their flesh. But there are ways to overcome grievous sin with the help of Jesus and by the power of His Word (the Bible). If we do not employ these methods, and become proactive, the blame only falls upon us for being lazy.
You said:
It very naturally leads to the Catholic system of mortal and venial sin with very particular details as to what is a sin that can cost you your salvation; and there is major problems with that system.
Just because Catholics are able to recognize the difference between grievous sin and minor infractions (or faults of character) in Scripture, does not mean that they get the entire subject matter exactly correct. Much of what they say is also based on their tradition and not solely Scripture alone. This is why their thinking on this is a bit muddied when it comes to the correct categorization of what sins can condemn and what sins do not condemn.
Minor infractions or
faults of character is another name for a "sin not unto death" (i.e. not unto death = not unto spiritual death or the second death) (Note: While the "sin not unto death" mentioned in 1 John 5:16 is in context to confessed sin in John's epistle (See: 1 John 1:9), it can be extended loosely to refer to the kind of sins that do not lead to spiritual death by it's very name).
"Who can understand his errors? cleanse thou me from secret faults."
(Psalms 19:12).
Examples of Sins That Do Not Lead Unto Spiritual Death:
#1.
1 John 5:16-17 mentions the "sin not unto death."
In context to
1 John 5: This would be talking about confessed grievous sin that one is striving to overcome with the Lord's help (1 John 2:1) (1 John 1:9) (Romans 13:14). In
1 John 5, the brethren are praying for this believer to have victory (life) over their sin as this believer confesses their sin.
#2.
Psalms 19:12 says, "Who can understand his errors? cleanse thou me from secret faults." I believe these would be faults of character or minor errors that a person might make with people. Example: A person may not be a great listener, and they do not allow others to speak like they should. By not listening to others may come off a little like they do not care (even though they care in their own way or a different way). Paul wanted to go to Jerusalem. The Spirit warned Paul not to go. He was still determined to go because he loved His fellow Jews and wanted them to know the love of Jesus. The Spirit told the brethren that Paul will be imprisoned by his going to Jerusalem. The brethren warned Paul not to go. They were in tears and loved him and begged him not to go. But Paul did not listen. He would not hear them. He did not want to hear it. Fault of character. Hidden fault. Minor error of his character. It is not something that condemned him (See
Acts of the Apostles 21, and read this article
here by Ray Stedman; Note: There is even a better write up than this one by Bible commentator James Boice
here; Please keep in mind I do not share their views on Soteriology, though; I merely agree with their view on what happened with Paul in
Acts of the Apostles 21).
#3. The Command to Be Baptized.
Paul says Christ sent him not to baptize but to preach the gospel (
1 Corinthians 1:17). If it was essential to salvation, then why would Paul say something like this? In
1 Peter 3:21: Peter says baptism is not for the putting away of the filth of the flesh. If you were to turn to
2 Corinthians 7:1, you would see that it uses similar wording ("filthiness of the flesh") that is clearly in reference to sin. So Peter is saying that baptism is not for the putting away of "sin" [i.e. filth of the flesh]. In other words, baptism is not a command that if disobeyed, leads to spiritual death.
#4. Other Commands in the New Testament that do not seem like a major violation of loving God and loving your neighbor that have no death penalties attached to them. One example would be the command to Rejoice when men persecute you or falsely accuse you of evil in
Matthew 5:11-12.
Real world examples: Going 5 miles per hour over the speed limit in area that is not life threatening to others. Not taking out the trash yesterday when it was a little stinky. These minor transgressions would obviously not send a Christian to hell.
But we as Christians strive to obey and do good in all things in the Lord. We strive to keep His commandments. For it is written,
"For this is the love of God, that we keep his commandments: and his commandments are not grievous.' (1 John 5:3).
Side Note:
Please understand that it is not possible for a person to obey God's commands without them first being saved by Jesus Christ and His grace (i.e. by seeking forgiveness of their sin with Him, and believing in His death, and resurrection on their behalf). For Christians are initially and ultimately saved by Jesus Christ.
And of course, you can never be sure of your salvation. You always could have sinned bad enough to lose it, contrary to what Jesus teaches in John 10:26-29. Or you judge yourself as good enough to not have lost it, which has its own problems.
No. You are not reading John 10:26-29 correctly. Verse 27 gives us the condition of not being plucked out of his hand in verse 28. Verse 27 says we have to FOLLOW Jesus so as to be the kind of sheep that cannot be plucked out of his hand in verse 28.
2. All that we are in Christ, all that salvation is, and we can just lose it? And gain it back again? And lose it again? We're a new creation, a royal priesthood, a holy nation, a child of God- and all that's out the window if we sin?
Yes. What do you think happened in the garden?
How do we become un-born again?
You cannot become unborn again spiritually, but you can most certainly can die spiritually (of which the Bible talks a lot about). The Parable of the Prodigal Son proves this fact. When the son came home to the father and he was willing to repent, the father said to him that he was "dead" and he is now "alive again." The parable is speaking in spiritual terms. He was dead spiritually when he was living in sin with his prodigal life, and he is now alive again spiritually in coming back home to his father and repenting (seeking forgiveness) with him (See Luke 15:11-31).
You said:
How do we go back from being a child of God to a child of wrath, and then a child of God again? How many times can you commit a serious sin and repent and what's the Scriptural evidence for it? It leads to the unmistakable conclusion that nothing God has done in our life has actually been done, but it's all potential, all waiting for us to maintain it. But Scripture says "For the gifts and the calling of God are irrevocable. (Romans 11:29)"
This is why we are to endure to the end, overcome, keep ourselves in the love of God. Fight the good fight of faith. If not, there is nothing to endure, overcome, or to fight for anything. All a person has to do is just kick back and eat their Cheetos on the couch and wait for Christ to return while others suffer and cry outside their door for the hope of Christ and his salvation to change their lives.
Jude 1:24 ESV
Now to him who is able to keep you from stumbling and to present you blameless before the presence of his glory with great joy
Jude 1:4 warns against those who turn God's grace into a license for immorality. So Jude 1:24 would not apply to such individuals. God keeps the believer who does not justify sin, and if they seek the help of God (via prayer) and they apply the power of His Word to their lives.
You said:
2 Corinthians 1:22 ESV
And who has also put his seal on us and given us his Spirit in our hearts as a guarantee.
Seals are not a bank vault or a padlock that cannot be broken. The seals mentioned in Scripture are actually said that they can be broken.
You said:
2 Timothy 2:19 ESV
But God's firm foundation stands, bearing this seal: “The Lord knows those who are his,” and, “Let everyone who names the name of the Lord depart from iniquity.”
How is that possible if one teaches that no sin can separate you from God?
If there are no true real consequences in committing in grievous sin, then one will make light of sin and not really make it all that important to overcome because all that is required to be saved is to believe in Jesus. No need to worry in doing much of anything if such is the case. For if there was no consequences in speeding on the highway then people would go at whatever speed they liked (even if there was a law saying not to go above a certain speed). But if a person knew they could be fined 50,000 dollars and get prison time for speeding 5 miles over the speed limit, I guarantee you that all speeding would either be greatly reduced or would be almost non existent.
