I want to point out to you a correct way of studying theology, for I've had practice in that. If you keep to it, you will become so learned that you yourself could (if it were necessary) write books just as good as those of the fathers and councils. This is the way taught by holy King David (and doubtlessly used also by all the patriarchs and prophets) in Psalm 119. There you will find three rules, amply presented throughout the whole psalm: prayer (oratio), meditation (meditatio), and affliction (tentatio).
First, you should know that the Holy Scriptures constitute a book that turns the wisdom of all other books into foolishness, because not one teaches about eternal life except this one alone. Therefore you should straightway despair of your reason and understanding. With them you will not attain eternal life, but, on the contrary, your presumptuousness will plunge you and others with you out of heaven (as happened to Lucifer) into the abyss of hell. But kneel down in your room and pray to God with real humility and earnestness (as David did), that he through his dear Son may give you his Holy Spirit, who will enlighten you, lead you, and give you understanding.
Second, you should meditate not only in your heart, but also externally, by actually repeating and comparing oral speech and literal words of the book, reading and rereading them with diligent attention and reflection, so you may see what the Holy Spirit means by them. Take care you do not grow weary or think you have done enough when you have read, heard, and spoken them once or twice, and that you then have complete understanding. You'll never be a particularly good theologian if you do that, for you will be like untimely fruit which falls to the ground before it is half ripe. God will not give you his Spirit without the external Word.
Third, there is affliction. This is the touchstone that teaches you not only to know and understand, but also to experience how right, how true, how sweet, how lovely, how mighty, and how comforting God's Word is, wisdom beyond all wisdom.
David, in Psalm 119, complains often about all kinds of enemies, arrogant princes or tyrants, false spirits and factions, whom he must tolerate because he meditates, that is, because he is occupied with God's Word in all manner of ways. For as soon as God's Word takes root and grows in you, the Devil will harry you and will make a real theologian of you, for by his assaults he will teach you to seek and love God's Word. I myself am deeply indebted to my critics, that through the Devil's raging they have beaten, oppressed, and distressed me so much. That is to say, they have made a fairly good theologian of me, which I would not have become otherwise. And I heartily grant them what they have won (honour, victory, and triumph) in return for making this of me, for that's the way they wanted it.
Now, with that you have David's rules. If you study hard in accord with his example, then you will also sing and boast with him, 'The law of thy mouth is better to me than thousands of gold and silver pieces,' (Ps. 119:72). And it will be your experience that the books of the fathers will taste stale and putrid to you in comparison. You will not only despise the books written by adversaries, but the longer you write and teach, the less you will be pleased with yourself. When you have reached this point, then do not be afraid to hope that you have begun to become a real theologian, who can teach not only the young and imperfect Christians, but also the maturing and perfect ones.
If, however, you feel and are inclined to think you have made it, flattering yourself with your own little books, teaching, or writing, because you have done it beautifully and preached excellently; if you are highly pleased when someone praises you in the presence of others; if you perhaps look for praise, and would sulk or quit what you are doing if you did not get it–if you are of that stripe, dear friend, then take yourself by the ears, and if you do this in the right way you will find a beautiful pair of big, long, shaggy donkey ears. Then do not spare any expense! Decorate them with golden bells, so that people will be able to hear you wherever you go, point their fingers at you, and say, 'See, See! There goes that clever beast, who can write such exquisite books and preach so remarkably well.' That very moment you will be blessed and blessed beyond measure in the kingdom of heaven. Yes, in that heaven where hellfire is ready for the Devil and his angels.
From LW, Vol. 34