The same way anyone learns anything true about God -- by the merciful, free gift of the indwelling Holy Spirit...
"But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you." -- John 14:26
"For it is God which worketh in you both to will and to do of his good pleasure." -- Philippians 2:13
How do we know Paul was correct? By the same manner, comparing Paul's words with the rest of Scripture, as we are led by the Holy Spirit. When we read Paul and realize the good fruit that comes with following God's word through Paul, our faith is confirmed, exactly as God has ordained.
Pray without ceasing. In every thing give thanks: for this is the will of God in Christ Jesus concerning you. Quench not the Spirit. Despise not prophesyings. Prove all things; hold fast that which is good. Abstain from all appearance of evil. And the very God of peace sanctify you wholly; and I pray God your whole spirit and soul and body be preserved blameless unto the coming of our Lord Jesus Christ. Faithful is he that calleth you, who also will do it. Brethren, pray for us. Greet all the brethren with an holy kiss. I charge you by the Lord that this epistle be read unto all the holy brethren." -- 1 Thessalonians 5:16-27"Rejoice evermore.
Very well. Did that gift indwell the eleven Apostles prior to any of the New Testament was written?
Truly, the "vital" teacher is God Himself: