Should new believers understand the deep things of the Bible right away?
No. The analogy is that new believers are like babes who are fed milk, not solid food, because they cannot comprehend all the deep things of God's Word at once. Growth is expected, but immediate understanding of deep truths is not.
Les explains that the word "carnal" in this passage refers to believers who have not really come out of the fleshly experience and are still under the control of the Old Adam rather than the Holy Spirit. They are saved, in Christ, but so weak spiritually that Paul speaks of them as babes. Paul says he fed them with milk, not solid food, which is an analogy to a newborn baby who cannot begin with solid food. Les says he has patience with new believers and does not expect them to suddenly comprehend all the deep things of God's Word. What he looks for is growth. He tells his classes that if they are not more steeped in the Word now than a year ago, then both he and they have failed, because God expects growth from His children. The Corinthians were still fleshly minded even though they were believers, born again, and placed into the Body of Christ.