But why would these newly regenerate be without the Spirit?
I don't see them as without the Spirit because they are regenerate (see 1Cor3:16). Paul has quite a bit of pneumatology in 1Cor from what I see. Using the NKJ a search of spirit* shows Paul speaking of it 57 times in this letter.
One of the discussions of what Paul means when he says "spiritual" is what he is saying in 2:6-16. Basically, the discussion is whether he is talking about regenerate vs. unregenerate or mature vs. immature Christians. I don't see the reason for the discussion. 1Cor3:1 is clear that he is using "spiritual" to contrast Christian infancy. One of the ways Paul speaks of the mature in Christ is with this word "spiritual" and one verse to take a look at is Gal6:1. His point there is that the mature in Christ who have been raised and trained to maturity have the ability to identify any sin, assist fellow Christians out of sin & thereby fulfill Christ's Law (think love one another as He loved us & gave Himself for us to save us from our sins).
In infancy and along the way to maturity (and thereafter to lesser degrees) we can grieve the Spirit, quench/extinguish the Spirit, not accept the [Apostolic] teaching of the Spirit and thus reject Him, and as Paul was saying to the infants, they can thus be fleshly, and walk according to man. So, they have the Spirit, but they in this sense do not have the Spirit - they in this sense have rejected Him - they do not have the deeper things of God Paul is saying in 2:6-16 the spiritual ones have - and they are walking as [unregenerate] men without [more of] the wisdom of God in strife and division and broken fellowship.
In 1Cor3 Paul goes into the Temple of God with the Spirit dwelling in you lesson. And he says the Temple can be defiled. He says if anyone of you thinks he is wise then become fools so you can become wise and then he clarifies that he is talking about worldly wisdom again as he was addressing in the first 2 chapters. IOW, the Christian can be deceived with the wisdom of the world, so he would then need to become a fool [to the wisdom of the world], so he can become wise to the wisdom of God taught by the Spirit, in this sense have the Spirit, now walk in Spirit, stop defiling the Temple. In the deceived & defiling condition the Christian indwelt with the Spirit of God is rejecting the Spirit, the Word, and is acting like the unregenerate as Paul spoke of in 2:14. I see this as part of the reason Paul spent so much ink in 1Cor2 teaching about the wisdom of the world and the unregenerate seeing God's wisdom as foolishness. The Christian can be deceived by the spirits and look and function no different than the unregenerate, especially in infancy. This is why we take so much care to protect and raise infants.
Being without the Spirit is thus something that needs to be clarified. Obviously being without the Spirit in the absolute sense is being unregenerate. One with the Spirit can
in a sense be functioning as without the Spirit - without the teaching of the deeper things of God - and when deceived, not be walking in Spirit. I think this is why Paul uses the word "spiritual" to speak of the more mature. The spiritual have more teaching of the Spirit and are living like it walking in Spirit not accomplishing the things of flesh (being fleshly). He also uses the word "perfect/mature" in Gal3:3, which is the word used in Heb5:14-6:1 for maturity. The Hebrews writer contrasts completion/perfection/maturity with infancy as Paul contrasts spiritual with infancy. Paul in a way links the spiritual to the perfect/mature in Gal and thus to the Hebrews lessons.
Was their "conversion" a human decision apart from the Holy Spirit? Was their faith counterfeit (
Mt 7:21-23)?
I don't see anything in conversion being done apart from the work of God. I think I'll take this up with Mark in the Monergism vs. Synergism discussion.
I think it's clear that Paul was treating them as regenerate but taking up the infancy problem discussed above that can look like being unregenerate. Could some of them still walk away as counterfeit? Probably, but I didn't look to see if he specifically addresses this in 1Cor.
He's most definitely working to resolve the infancy problem (and other problems) and in 1Cor3 he's essentially warning both the building (the Christians - the Temple of God) and the builders that they best be building the Temple with quality products and workmanship. Paul says in 1Cor3:11 that he placed the only foundation that can be placed - Jesus Christ (and recall he earlier said his earlier focus for various reasons among these Christians was Jesus Christ and Him crucified).
This is his point when we look at all that's going on in Corinth. He knows he placed the only foundation properly. He knows they (most?) accepted it. He knows he could only take them so far at the time. He knows there are infancy problems there and that his authority and thus teaching per the Spirit is being challenged. He is dealing with a lot and working to overcome a lot to get the deeper things of God taught by the Spirit into that congregation. In this sense he is working to get the Spirit [more] into that group and get them to the "spiritual" condition.
We have a historical precedent for our background that shows us at least what direction to go in on matters.
It's not all wide open like a fairy tale.