13 These things we also speak, not in words which man’s wisdom teaches but which the Holy Spirit teaches, comparing spiritual things with spiritual.
14 But the natural man does not receive the things of the Spirit of God, for they are foolishness to him; nor can he know them, because they are spiritually discerned.
15 But he who is spiritual judges all things, yet he himself is rightly judged by no one.
16 For “who has known the mind of the Lord that he may instruct Him?” But we have the mind of Christ.
these things (v.13) — the things revealed by God (v.12)
which man’s wisdom teaches (v.13) = which are taught by man’s wisdom
comparing spiritual things with spiritual (v.13) — This refers to the human interpretation of divine inspiration. What is divinely inspired must be humanly interpreted. But how? By “comparing spiritual things with spiritual.” That is, using spiritual methods. The communication of spiritual things must be by spiritual methods. This requires spiritual men. Thus, spiritual men and spiritual methods will bring to light the wonder of spiritual words. — Laurin, apge 62.
The best commentary on the Bible IS the Bible — a book (singular) made up of sixty-six books (plural); and each book is necessary if we would understand the program of God from beginning to end … The best way to understand the Bible is to compare Scripture with Scripture, spiritual things with spiritual. We should never take Scripture out of its setting and use it to prove a denominational or religious point. We should never add to it what we think, and we should never take away from it to make it say what we want it to say. We should study and rightly divide the Word of Truth as the Holy Spirit leads us. — Greene, page 111.
Verse 13 is saying that human learning and understanding is incapable of expressing divine truth.
natural (v.14) — what pertains to man — merely human
know (v.14) = get to know — Man’s wisdom can’t even get partway to God
The Bible divides all humanity into two classes, and only two — saved and lost. The lost man is called the natural man because he has been born only once of natural parents, through the line of Adam, man’s federal head. The saved man is called the spiritual man, because he has been born again from above the second time, by the Spirit of Almighty God Himself. The natural, unregenerate man possesses the nature of father Adam. The spiritual man possesses the Spirit of God, having become a partaker of the divine nature (2 Peter 1:4). — De Hann, page 27.
discerned (v.14) = distinguished so as to form an opinion
spiritual (v.15) — the man in Christ
judged by no one (v.15) — The man in Christ is a riddle to the man without Christ.
Verse 16 is a quote from Isaiah 40:13 in the Septuagint (see Romans 11:34).