These are all great questions, Thomas. You ask these in response to...
To answer your first question, this is not a hard statement to understand. Nowhere in Scripture do we find anyone having faith that did not first have the will of God revealed to them. This began in the Garden. God gave man His will for his life and Adam responded. I think the next questions will serve well to expand on the meaning of the statement.
God is only immanent in those who have been born again, and this did not begin until Pentecost. And even for the Regenerate we see His intervention. The primary problem of man is that he is born separate from God and thus it is only God that can reveal truths to men by which men can be saved. The truth revealed to Adam was "The day you eat of this fruit you will die." Adam's response was wilful disobedience, thus death entered the world (death, not sin). We see this pattern throughout Scripture and in the case of everyone (which is everyone, lol) who comes into relationship with God. We do see the work of the Holy Spirit in the Old Testament, and that He would come upon men and women to empower them for ministry (i.e., Prophet, Priest, King, Warrior, etc.) but that is distinguished from the eternal indwelling of God which was promised in the Old Testament and brought about by Christ's death, burial, and Resurrection. When He returned to Heaven He sent the Comforter to perform a ministry specific to this Age:
John 16:7-9 King James Version (KJV)
7 Nevertheless I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you.
8 And when he is come, he will reprove the world of sin, and of righteousness, and of judgment:
9 Of sin, because they believe not on me;
When the Comforter comes He will convict men of sin...because they are unbelievers. They do not yet believe in Christ, and it is His ministry by which they come to believe. Few notice that the disciples of Christ, though ministering under Him for three years...did not believe the Gospel. Peter rejected the Gospel when the Lord told it to him...
Matthew 16:20-23 King James Version (KJV)
20 Then charged he his disciples that they should tell no man that he was Jesus the Christ.
21 From that time forth began Jesus to shew unto his disciples, how that he must go unto Jerusalem, and suffer many things of the elders and chief priests and scribes, and be killed, and be raised again the third day.
22 Then Peter took him, and began to rebuke him, saying, Be it far from thee, Lord: this shall not be unto thee.
23 But he turned, and said unto Peter, Get thee behind me, Satan: thou art an offence unto me: for thou savourest not the things that be of God, but those that be of men.
Then we see Peter seek to keep Christ from the Cross by physical violence towards men in the Garden. Then we see Peter deny he even knows Christ. Then we see Peter (and the other disciples) disbelieve He had in fact risen again. How this is relevant is that the Gospel of Jesus Christ was the Hidden Wisdom of God, kept secret from the foundation of the world. It was a Mystery, previously unrevealed truth. While men could place faith in a coming Messiah/Christ, that is not to be confused with believing on and placing faith in the Risen Savior, that He died for you. So the distinct ministry of the Comforter is unique to this Age, and it is through receiving the Gospel that men are reconciled to God. They are not only brought into relationship with God, but one that is eternal. We are made one with God. So while truth was revealed to men in the Old Testament, and man's response to those truths determined their standing with God (as well as created belief and faith), today the revelation being provided men is the revelation of the Gospel of Jesus Christ.
This is how Scripture describes man as he comes into this world. The natural man (unsaved) is contrasted with the spiritual man (saved:
1 Corinthians 2:9-14 King James Version (KJV)
7 But we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory:
8 Which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory.
9 But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him.
10 But God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God.
11 For what man knoweth the things of a man, save the spirit of man which is in him? even so the things of God knoweth no man, but the Spirit of God.
12 Now we have received, not the spirit of the world, but the spirit which is of God; that we might know the things that are freely given to us of God.
13 Which things also we speak, not in the words which man's wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual.
14 But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned.
I post from v.9 because it is important to understand the context of this passage. Most view v.9 as speaking about how great Heaven is going to be, but this quotation is referring to the Hidden Wisdom of God...the Gospel of Jesus Christ. No man had an understanding of the Sacrifice of Christ. that His death is in view is stated before:
1 Corinthians 2 King James Version (KJV)
1 And I, brethren, when I came to you, came not with excellency of speech or of wisdom, declaring unto you the testimony of God.
2 For I determined not to know any thing among you, save Jesus Christ, and him crucified.
That is the central focus of not only the second chapter, but the latter half of chapter One. The point being that it is the Spirit of God that has revealed to us this knowledge, and that it was not revealed previously to men. Again we see the simple pattern: God reveals truth, man responds. The natural man cannot understand spiritual things hence it is God that enlightens his mind that he may respond. Apart from that intervention he would remain blind to the truths he desperately needs to know. When one receives those truths and is converted he stops being a natural man and becomes something he wasn't before, what Paul calls the new creature.
Primarily we see the will of God for man revealed. All revelation has a primary goal of bringing man into obedience to God's Own will, which ultimately is for the good of man.
How revelation is provided has, in my view, three primary avenues: First, we see revelation provided in Creation itself (Romans 1:20). Since the world began men have been able to understand there is a God Who created the world.
Second, we see an internal witness provided to men, in which God reveals to man's conscience His will (Romans 1:19-20; Romans 2:13-15). That is what we would view as immanent.
Third we have direct revelation, which occurs when God speaks directly to men, through men, or through the written Word.
All revelation must be kept in the context in which it is given. For example, the revelation provided to men under the Law cannot be imposed as specific to our Age. If a Gentile tries to "keep the Law" he ignores that the Law was given specifically to Israel and that there are certain aspects of the Law they were commanded which is impossible for Gentiles to keep.
A great example of revelation being kept in its context would be the Gospel of Christ. It is given in the Old Testament but the understanding of it was not given. Abraham believed God would give him a son and through his seed (offspring) all families/nations of the earth would be blessed. He believed it and had enough faith in Gd's promise that he was willing to slay his son, knowing that God would have to resurrect him (Isaac) in order to fulfill His promise. But he didn't understand that God would manifest in flesh, die in his stead for his sins, and bestow eternal life. Getting back to the OP, because of his belief and faith he was justified. He was declared righteous on a temporal basis, and this secured his eternal destiny. But he would not be justified through the redemption which is in Christ Jesus...until Christ actually redeemed him.
Adam and Eve also received the Gospel:
Genesis 3:15 King James Version (KJV)
15 And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.
But neither did they understand that the Son would die for their sins. It is not until the Comforter begins His convicting ministry that the Gospel is revealed to men. Notice that the disciples of Christ did not preach the Gospel, nor did the Church begin...until Pentecost.
Well, believing is something both saved and unsaved can do. The devils believe...and tremble. People can believe Jesus is the Christ is the Son of the living God, but as we see in Matthew 16 this does not equate to believing on Christ in faith. It is just my opinion that many in the churches today engage in the "faith" the disciples had prior to Pentecost: they believe Christ is Who He says He is, but they have not yet placed faith in Him as Savior.
As to the difference of the two in my statement, God reveals to men truths, they believe, but just because they believe doesn't mean they have faith. Abraham proved he believed by being willing to offer up his son. The Apostles proved their belief by being willing to die for Christ. Just because someone believes in something doesn't mean they have faith in it. I believe Satan is a powerful demon but I have no faith in him.
In a salvific context placing faith in Christ is not just a matter of believing. Many Jews all over the world trust in the Messiah of the Hebrew Scriptures, but as evidenced by the actions of the disciples (and all men) during Christ's ministry, the "faith" they had was shallow and weak. Consider...
Galatians 3:23-25 King James Version (KJV)
23 But before faith came, we were kept under the law, shut up unto the faith which should afterwards be revealed.
24 Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith.
25 But after that faith is come, we are no longer under a schoolmaster.
Paul makes it clear that faith in Christ is specific and contrasted to faith in general. While men were under the Law (and the implication being prior to the Law as well) faith was not possible. No man under the Law had the Faith of Christ. Many believed in the Messiah that was prophesied (such as the woman at the well, John 4), and as I said many Jews today believe in that Messiah, but...
...only those who believe and place their faith in Christ have received the faith which was not possible under the Law.
In our culture it is not as evident what faith in Christ looks like, but in the First Century it was impossible to miss. Men and women were persecuted for their faith, so faith would have "looked like" men and women dying, being shut up in prison. In our culture it might look like regular attendance with a group of other Christians. In a general statement, for me, faith looks like keeping the commandments of Christ. This is not to be confused with keeping the Law, or keeping the teachings He gave in specificity (i.e., "do unto others..."), but generally keeping everything He taught and viewing it as irrefutable. This would include His doctrinal teachings. Many today, for example, do not "keep" His teaching concerning Regeneration. In John 3 Nicodemus asks how men can be born again and the Lord's answer is He must be "lifted up," or in other words die on the Cross. Yet no-one "keeps" that teaching.
In another sense we might say that observing what placing faith in Christ is might be difficult. While I believe we can see evidence of genuine salvation, we (humans) have the bad habit of judging books by covers, as well as demanding more of others than we do of ourselves. We see someone smoking, for example, and we question their faith. But faith in Christ begins a growth process, so all believers are at differing phases of maturity in Christ. Genuine faith in Christ is expressed according to that maturity so it might be difficult to say for certain whether one is saved or not.
Okay, sorry for the length, but as I said, great questions. Thanks, much for them.