I have a question... and I need help.
What does the bible say about what a father must do if a son (almost of legal age) becomes completely deceived by Mormonism... even to the point of expressing a level of faith that cannot be changed by reason and examination?
First - you should pray for your son. Second, you need to read up on Mormonism and understand what the differences are between LDS and Christianity. Once you have found some excellent resource material exposing the LDS faith for what it is, challenge your son to read it. Start with Utah Lighthouse Misitries to find good resource material.
Well, the apostle Paul was single, but he treated those the Lord used him to win as if they were his spiritual offspring:
Even though you have ten thousand guardians in Christ, you do not have many fathers, for in Christ Jesus I became your father through the gospel. Therefore I urge you to imitate me. (1 Cor. 4:15-16)
I don't know how much of the true gospel your son accepted or rejected, but if that was something you clearly shared with him, remind him that you are twice his father -- once naturally, and once supernaturally.
And what you have been going through, is what Paul went through with the Galatians -- the “father heart cry” of Paul for the Galatians was, “My little children of whom I labor in birth AGAIN UNTIL CHRIST IS FORMED IN YOU." (Gal. 4:19)
What was the bottom line issue with the Galatians? Paul sums it up in Gal. 3:3: Are you so foolish? After beginning with the Spirit, are you now trying to attain your goal by human effort?
You need to show clearly the difference between the Christian lifestyle and that of legalistic Mormonism (2 Nephi 25:23) -- "you are saved by grace AFTER ALL YOU CAN DO."
Your son, nor any Mormon, can ever do enough. Not physically. Not emotionally. Not spiritually. Not relationally.
And that's why, as you read Gal. 4:19 and following, you see Paul immediately describe those in the OT trying to live by the law -- those who became Muslims. Paul describes those as illegitimate sons -- much like Hagar's child.
You need to somehow convey to your son, that his Father in heaven sees those who try to earn their sonship as taking the illegitimate course.
We don't earn our Father's sonship; we are sons. That's the difference also between the Mormon false "gospel" and the true one: The Mormon bad news is always, "thou shalt perform" -- so as to be deemed worthy in God's eyes -- as if God didn't already know our inward heart.
The Christian gospel says, you are forgiven. Therefore, live as one forgiven. (Our deeds then simply become the by-product of a forgiven lifestyle).
I would make a suggestion that you save up for him the nicest gift you could give him either for Christmas or his 18th birthday. After you give it to him, in all seriousness, hand him a written out payback plan for that gift.
Diversify the payback plan. Make some of the "payments" work he has to do for you; make some of the payments (10%) actual $; make some of the payments behavior-based; etc.
Don't go to the punchline too early on this. Show him you are dead serious (this isn't a joke). Let it linger. Don't help him out of this struggle right away.
The punchline: When you do reveal the punchline to him, give him a written list of all the requirements in Mormonism...
all their ordinances;
precept keeping;
commandment keeping;
their temple recommend checklist-keeping;
etc.
Mormon leaders' quotes re: how they see their life as a "payback" to God -- as if He could be paid back....
Then contrast that with the historic Christian understanding of redeemed -- how it literally means to be bought back. How the phrase Jesus said on the cross, "It is finished" -- was also a phrase used in their time to mean, "paid in full."
And then explain that if any child tried to insist on paying back a wonderful gift from their father, that child would eventually insult the father. A gift is a gift is a gift. (Eph. 2:8-9; Rom. 6:23 explain how our eternal life is a gift -- not a payback plan)
And then tell your son he's embarking on a grand payback scheme toward a gift via Mormonism, and how in the process, he's insulting our Father in heaven. A gift from our Father is a gift is a gift.
Two reasonable people equally informed, seldom disagree. If you want to help your son you must become equally informed about the LDS church. Have you read the Book of Mormon? Have you prayed and asked God if it is true?
The Bible say If any of you lack wisdom, let him ask of God.
The Bible says that there will be both true and false prophets in the last days. By their fruits you shall know them. The true church of Jesus Christ will be led by prophets that receive revelations from Him.
We have 3 choices today. We can follow true prophets like Joseph Smith and the prophets that followed after him. They will lead us to Jesus. Or we can follow false prophets that will lead us away from Jesus. Or we can follow no prophets which will result in our not being led to Jesus.
We do have choices. But choose wisely.