Part of the once saved always saved theology is the once a son always a son argument. 
To the grace changers, to reason from the natural to the spiritual without Scriptural backing means nothing, as long as such can somehow be used to support eternal security. Since once a son always a son is natural reality, then it must also be spiritual reality as well, according to them!
For all those who are wise enough to cleave to the Biblical message for all doctrinal truth (2 Tim. 3:16,17), the once a son always a son idea unravels in several ways. Gal. 4:7 does it, but not as directly as dozens of others verses. Here is the word of God and spiritual reality:
So you are no longer a slave, but a son; and since you are a son, God has made you also an heir. (Gal 4:7)
If one becomes a son of God, he also becomes an heir. That means he has a spiritual inheritance. By implication from Gal. 4:7, if one’s spiritual inheritance can be lost, then one’s spiritual sonship can also be lost at the same time. If that occurs, then there is no once a son always a son, as implied by that Baptist doctrine. So can that terrible thing occur? Absolutely and here is the Scriptural proof:
Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals, nor thieves, nor the covetous, nor drunkards, nor revilers, nor swindlers, will inherit the kingdom of God. (1 Cor. 6:9,10, NASB)
For of this you can be sure: No immoral, impure or greedy person–such a man is an idolater–has any inheritance in the kingdom of Christ and of God. Let no one deceive you with empty words, for because of such things God’s wrath comes on those who are disobedient. (Eph 5:5,6)
The acts of the sinful nature are obvious: sexual immorality, impurity and debauchery; idolatry and witchcraft; hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions and envy; drunkenness, orgies, and the like. I warn you, as I did before, that those who live like this will not inherit the kingdom of God. (Gal 5:19-21)
The saints inherit the kingdom of God, but before final salvation that inheritance can be lost through certain types of sin, according to 1 Cor. 6:9,10; Eph. 5:5,6 and Gal. 5:19-21. Christians were warned about losing their inheritance because it could happen! That is another way of saying that a saint, because of certain types of sin, can become unrighteous, die spiritually and lose his spiritual sonship. Therefore, once a son always a son might be based on natural fact, but it is not spiritual reality and contrary to Scripture. BTW, that is a good way to summarize the whole eternal security theology, that is, it is not based on spiritual reality and contrary to Scripture!
[Since the once a son always a son argument has been dealt with at other points in The Believer’s Conditional Security they will not be rehashed here. Click this link to purchase that NO eternal security book.]
