I recently got an email that read as follows:

I am studying with Koinonia Institute and have recently encountered a teaching that is making me feel very uncomfortable. I am being a Berean about this teaching and am therefore looking for some help from you if you can possibly provide some aid.
The teaching is that 1) The Kingdom of God is a separate entity from The Kingdom of Heaven. Additionally, we are being taught that 2) the Bride of Christ is only a remnant of the Body of Christ, thus we need to 3) work out our sanctification in order to inherit “the Kingdom.” Which kingdom, I am not sure. It is being claimed that our inheritance of salvation (justification) is blood bought and cannot be lost. I agree. However, the teaching that we must work for a second inheritance to rule and reign with Christ is not sitting well with me. I do believe that we are to allow the Holy Spirit to sanctify us, but I do not believe that we will “lose” our inheritance. Do you? I do believe we can earn or lose rewards–but not an inheritance. And finally, 4) a lot of Christians will be disappointed when they get to heaven because they will have forfeited their inheritance. Thus Scripture tells us there will be weeping and gnashing of teeth, but the God will wipe away every tear.
Pardon my sigh. All I could think of was here we have yet another exegetically baseless “insight” that turns into some new doctrine. I thought readers might be interested in my reply (it’s short – promise).
The phrases kingdom of heaven and kingdom of God are interchanged in the synoptic gospels (e.g., Matt. 3:2 = Mark 1:15 / Matt 11:11 = Luke 7:28), so a distinction is bogus. The OT and Second Temple antecedent literature does the same. In Daniel, for example, there is talk of God’s kingdom interchanged with “heaven rules” and similar phrases. In a nutshell, the phrase “the kingdom of heaven,” is a Jewish circumlocution (roundabout way of speaking) for “the kingdom of God” in order to avoid unnecessary use of the word “God” (cf. Mark 11:30–31; Luke 15:18, 21). See here and here. The one by Davies and Allison is more technical but much shorter.
One cannot commit a sin that results in the loss of salvation. Salvation is not about moral perfection, hence it cannot be lost by moral imperfection. That said, those who profess to believe can turn their backs on God and choose another god or no god at all. Election does not rectify this situation, as Israel was elect, but many (most?) of the Israelites fell into idolatry. The key here is to realize that election and salvation are not synonyms. I’ve blogged about that before.
Think of it this way. With respect to Israelites, there are no Israelites in heaven whose loyalty was thrown to another god. Their “profession” means nothing if they were disloyal to Yahweh and worshipped another god (or no god at all). One had to believe in the true God. That belief was shown by spiritual loyalty to him, manifest by refusing to worship any other. Salvation in the OT was about believing loyalty. Same for the NT, since Jesus is this same God incarnate. No one will be in heaven who didn’t believe or who chose another God/Savior/faith. Likewise, there are no believers in Jesus in hell. The issue is believing loyalty directed exclusively at Jesus.This doesn’t resolve the question of people who make professions and then do not believe – as to whether they lost salvation or were never saved in the first place. I consider the question irrelevant. The issue is whether someone believes exclusively in Jesus or doesn’t. I don’t really care about their history. Whether someone had a profession or not prior to that question being raised is irrelevant. The solution is the same: believe on the Lord Jesus Christ, for there is salvation in no other name (Acts 4:12).