“For he taught them as one having authority, and not as the scribes.” (Matt. 7:29)
It is not easy to master a subject so well that all will attend to what you say, you must have a complete grasp of all the minutia of that subject, you must understand fully everything that it encompasses.
Whether He was twelve years old or a full-grown man in His three plus years of public ministry, the Lord Jesus Christ was the master of the subject He brought to the people, the Living God. We will never be that adept at expounding upon the entire Word of God, but there are some items you should become exceptionally proficient in.
The first of course would be the Sovereignty of the Almighty, this is in the greatest sense the foundation of all we have to offer to people. You must be able to show others through the Scriptures the truth that the Living God created all that is, all known reality was by His making. “Thou art worthy, O Lord, to receive glory and honour and power: for thou hast created all things, and for thy pleasure they are and were created.” (Rev. 4:11)
Unless this foundation is laid, everything built upon it can be questioned, the authority of everything is in the hands of God. This must be first.
His Holiness would be second, for the third will be the sinfulness of man, and unless you help those you are speaking to, to understand the perfect righteousness, the unfathomable Holiness of the Lord our God, the question of personal sin cannot be broached.
The answer to the sin question must be next, how it came upon all mankind and the punishment due because of sin, for if that is not expounded upon properly from the Scriptures, the next truth will not be understood, and that truth is grace.
Here you must spend much time, for 1st Peter 1:12 shows you the great mystery of grace. “Unto whom it was revealed, that not unto themselves, but unto us they did minister the things, which are now reported unto you by them that have preached the gospel unto you with the Holy Ghost sent down from heaven; which things the angels desire to look into.”
If the angels have what we could best describe as difficulty understanding grace, then we mere mortals will need much assistance in describing it. Here the Father offers us simplicity, here He shows us His beloved Son and says, “That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.” (Romans 10:9)
These are the subjects you must study, and studying them will show you approved, and bring glory to the Lord our God.
The cross is the end of this task, for it is at the cross where the Sovereignty of the Almighty is revealed, where sin was placed on His Son, where grace can be found, where the One whom all our hope and faith, all of love and devotion hung.
You must know these truths; you must be able to expound on them.
“But sanctify the Lord God in your hearts: and be ready always to give an answer to every man that asketh you a reason of the hope that is in you with meekness and fear:” (1st Peter 3:15)
You do not need to have the Ten Commandments memorized, the journeys of Paul, you do not even need to know every specific item of how one who has been born-again is to live their lives for the glory of God, but you need to know these truths.
His Sovereignty, His Holiness, our sin, His grace.
The cross and the reason the Lord Jesus Christ went there.
There is no subject that the Lord is not well versed in, He knows everything about everything and everyone. All you need to know is how to lead others to Him. Study on how you are expected to live your life for His glory, research the Scriptures on what it means to be a profitable servant, but be ready and willing at a moments notice to speak those truths that will lead others to Him.
