All that Paul is saying is that they use inspired speech ( i.e. Moses and the prophets as well as the revelations the Apostles themselves received, and that we now have in our Bibles ) to teach the spiritual realities of the Gospel, beginning with the "message of the Cross,"
1 Corinthians 1:18 , which is foolishness to those who are carnal, and a message that requires faith to receive.
That's not all Paul is saying.
1 Corinthians 1:17-1 Corinthians 2 addresses the Mystery of the Gospel. Thee is more than just the truth that we, as believers, understand the spiritual things of God because it is God enlightening us to those truths.
There was spiritual truth that was not revealed to the prophets that has been revealed to us, and that is what Paul is explaining in detail in 1 Corinthians 2.
The Bible contains the truth about the Truth, according to the verses you're citing, but there is no biblical doctrine that saves us. It is a Person that saves us.
According to the Bible, we are saved by the words of the Bible:
Acts 11:13-18 King James Version
13 And he shewed us how he had seen an angel in his house, which stood and said unto him, Send men to Joppa, and call for Simon, whose surname is Peter;
14 Who shall tell thee words, whereby thou and all thy house shall be saved.
15 And as I began to speak, the Holy Ghost fell on them, as on us at the beginning.
16 Then remembered I the word of the Lord, how that he said, John indeed baptized with water; but ye shall be baptized with the Holy Ghost.
17 Forasmuch then as God gave them the like gift as he did unto us, who believed on the Lord Jesus Christ; what was I, that I could withstand God?
18 When they heard these things, they held their peace, and glorified God, saying, Then hath God also to the Gentiles granted repentance unto life.
In this passage we see it is the very Gospel of Jesus Christ that God uses, through Peter, to save Cornelius. In Acts 10 we see Peter preach the Gospel and he (Cornelius) is Baptized with the Holy Ghost as a result. Meaning he (Cornelius) received the promise of the Father (Acts 1:4-5) just as they (the disciples at Pentecost) did.
I mention this, not because I am in complete disagreement with what you have been saying, but to point out that Revelation has always been progressive, and the spiritual things of God have been progressively revealed to the saints of God. This is, in my view, also true in the Body of Christ in this Age: there are those among the Body who are infantile in their understanding and those who are eating meat.
It's not that we can know a truth beyond the doctrines of the Scriptures,
It is true that all truth we embrace should be founded in Scripture and its Doctrine, but when we look at the progression of Revelation we see understanding given in more detail as time goes by.
The Gospel was revealed to Abraham in veiled form, that Abraham would have a son and that all families of the earth would be blessed through that son. Because the Spirit Paul is speaking about in 1 Corinthians 2 (the Comforter) has revealed to us what was not revealed to men then (eye hath not seen, ear hath not heard, nor has it entered into the hearts of men that which I have prepared...). we can look at that veiled Gospel and understand it as the Gospel of Christ as Abraham could not:
"Abraham, the Son that you shall have will be the Seed through Whom all families/nations of the earth will be blessed through."
Both are the Gospel, but we know a truth beyond the truth Abraham received. Men can believe Christ died to save intellectually, but that doesn't mean they view it as "spiritual truth."
Again, I think we can see this in the Body today.
it's that we can be known by the Truth while yet ignorant.
This is very true.
But both the "truth" that is in view as well as Who we receive and come to understand it by are two entirely different things for the two groups used as an example above.
But we won't remain there, for the Spirit guides us into all truth.
John 16:13.
I think there is going to be a minimum amount of knowledge that the Comforter is going to bring conviction concerning in the hearts of those yet unbelieving in Christ. The Comforter glorifies Christ in His ministry, and brings conviction concerning sin, righteousness, and judgment. "Of sin," Christ said, "because they believe not on me."
So no one, in my understanding of salvation, is going to be saved apart from understanding that Jesus is the One that will save them. Only an understanding of the Gospel is salvific, therefore.
Those are the words by which Cornelius was saved, and are the words by which all of us are saved.
God bless.