@St. SteVen I believe that your question is answered by history......when have God's worshippers NOT been exclusive and a closed system?
The Israelites, even in Egypt were kept separate, and after their release they were to receive God's Laws, which they were bound to obey from birth. God did not give his laws to any other people. Those who wanted to serve the one God of Israel, had to become "Jews" in practice before they were accepted as brethren. But they could not hold sacred office such as priests because this was exclusive to the Levites. God dictated their worship in every detail. They were not to introduce anything else, or to add to what God had commanded them...a closed system.
Jesus castigated the Pharisees because he said....
“Woe to you, scribes and Pharisees, hypocrites! because you shut up the Kingdom of the heavens before men; for you yourselves do not go in, neither do you permit those on their way in to go in. . . .
15 “Woe to you, scribes and Pharisees, hypocrites! because you travel over sea and dry land to make one proselyte, and when he becomes one, you make him a subject for Ge·henʹna twice as much so as yourselves."
There it is....one had to be "Jewish" by conversion in order to become a "Proselyte"....that meant coming under Jewish law the same as Israel was, but by their own choice, not by birth....again indicating a closed system.
Those who opposed Jesus, opposed his Father, and so he said "Gehenna" awaited them. This was no burning "hell" however because the ancient Jews never believed in life after death. There was no immortal soul ever mentioned in their scripture and they had the writings of the Hebrew scriptures to inform them that God's plan for the future was all about resurrection, not continuing life in spirit form whilst everyone on earth was still making up their minds about worship.
The Christians were likewise exclusive as far as their beliefs were concerned...all had to adhere to one truth which was taught by Jesus and those he trained to continue on in his absence. There was to be no divisions and no disagreements among them. (1 Cor 1:10) God's spirit would unify them. It was also a closed system. But as Jesus warned, the same enemy who corrupted the Jews, would also corrupt Christianity. History tells the sad and sorry tale of how that happened....and why we are seeing a very different "Christianity" to what Jesus and his apostles began almost 2000 years ago.
When Jesus comes to judge the world, they don't all get a free pass based on Jesus' sacrifice, because he did not die for the unworthy, the unbelievers, or those who wanted to use their faith to justify or excuse wrongdoing.....only those who lived their faith...not just those who merely talked about it....would qualify for life. There are only two categories of human beings on this planet at the harvest time....(which is now looming)..."sheep and goats". The "sheep" live....the "goats" are dispatched into everlasting death. (Matt 25:31-34, 41)
Again, the apostle Peter said.....
"Certainly if after escaping from the defilements of the world by an accurate knowledge of the Lord and Savior Jesus Christ, they get involved again with these very things and are overcome, their final state has become worse for them than the first. 21 It would have been better for them not to have accurately known the path of righteousness than after knowing it to turn away from the holy commandment they had received. 22 What the true proverb says has happened to them: “The dog has returned to its own vomit, and the sow that was bathed to rolling in the mire.”
So the "first state" mentioned here, was the state of being willfully ignorant.....not wanting to know about salvation through Jesus.
But God hates hypocrisy more than he hates ignorant complacency, and so he views those who know, and yet do not obey his son, as worse than those who don't know because they don't want to.
Hebrews 6:4-6...
"For as regards those who were once enlightened and who have tasted the heavenly free gift and who have become partakers of holy spirit 5 and who have tasted the fine word of God and powers of the coming system of things, 6 but have fallen away, it is impossible to revive them again to repentance, because they nail the Son of God to the stake again for themselves and expose him to public shame."
I cannot see the Bible as endorsing anything but a closed and exclusive system of worship that was to remain pure despite the corruption that Jesus foretold. The "wheat" were always in the world, but overshadowed by the "weeds" who took mankind into captivity as Israel had been taken into exile in ancient Babylon. The faithful did not adopt the ways of Babylon despite their 70 year sentence. Upon their release, God took them back to their homeland, re-established his worship and rebuilt his Temple, giving us types and shadows of what was to come.
In this "time of the end", God was going to 'cleanse, purify and refine' his worshippers (Daniel 12:4, 9-10) so that they could carry out the greatest preaching campaign the world has ever seen.....this is the only testimony that people will get....and it was to be global....taken out to "the entire inhabited earth" he said.....
"And this good news of the Kingdom will be preached in all the inhabited earth for a witness to all the nations, and then the end will come."......(Matt 24:14)
Because that "witness" has been given all throughout these last days, and "the end" is now in sight as we see the world systems collapsing, the judgment is not too far off. (Matt 7:21-23) Who will stand and who will fall? Jesus will let us all know.