Faulty premise; Jesus didn't gain omniscience until after his death. As for why God didn't lay down one definitive scripture, and attach a big glowing sign to it saying "this is the truth"... well, that's a good question.

As far as one "definitive scripture", is that what we really need? Remember the warning that Jesus gave at the end of the illustration of Lazurus and the rich man? Jesus said there, "If they do not listen to Moses and the Prophets, neither will they be persuaded if someone rises from the dead." see Luke Chapter 16 God will not provide special signs or miracles to convince people. They must read and apply the Scriptures if they would obtain his favor. Jesus did not take the inititive to do this, ( provide one definitive scripture),as it evidently was not his Father's will. Jesus always put his Father's will first. Therefore, in answer, Jesus went on to say to them: "Most truly I say to YOU, The Son cannot do a single thing of his own initiative, but only what he beholds the Father doing". John 5:19 "I cannot do a single thing of my own initiative; just as I hear, I judge, and the judgment that I render is righteous, because I seek not my own will but the will of him that sent me." "Because I have come down from heaven to do, not my will, but the will of him that sent me." And again: "My food is for me to do the will of him that sent me and to finish his work." see John 4:34; 5:30; 6:38, NW. No, "God Himself" did not write any of the Sixty-six books of the Bible. As The Apostle Peter said at II Peter 1:19-21; "Consequently we have the prophetic word [made] more sure; and YOU are doing well in paying attention to it as to a lamp shining in a dark place, until day dawns and a daystar rises, in YOUR hearts. 20 For YOU know this first, that no prophecy of Scripture springs from any private interpretation. 21 For prophecy was at no time brought by man's will, but men spoke from God as they were borne along by holy spirit." Moses, who compiled the Torah (the first five books of the Bible) "wrote down all that God told him to, which included the account of creation, the record of the Flood of Noah's day, and the history of Abraham and of Moses' own dealings with God. (Exodus 24:3, 4) King David said: "The spirit of the LORD has spoken through me, his message is on my tongue." (2 Samuel 23:2) Other Bible writers made similar claims of divine guidance. All these writings together form God's own explanation of history,its true meaning, interpretation, and final outcome. Consider the different backgrounds and the diversity of the writers of the Scriptures ;kings, hired laborers, priests, and others. They also included state officials, judges, prophets, shepherds, farmers, and fishermen,some 40 men in all. Thus, the Bible, although a message from God, has the warmth, variety, and appeal of the human touch. All of these men were acting as secretaries in recording the thoughts of God, the Bible's Author and the Guarantor of its promises. The Apostle Paul told Timothy at II Timothy 3 :16: "All Scripture is inspired of God and beneficial for teaching, for reproving, for setting things straight, for disciplining in righteousness, 17 that the man of God may be fully competent, completely equipped for every good work&". Should this not prompt us "to pay more than the usual attention" to this extraordinary book, the Bible? Should we not be able to reach the same conclusion as did Peter, who wrote: "All this only confirms for us the message of the prophets, to which you will do well to attend, because it is like a lamp shining in a murky place" see Hebrews 2:1; 2 Peter 1:19, The New English Bible. Yes, the Bible is under attack, but it has been preserved through the ages. "As far as is known today, no handwritten original, or autograph, manuscripts of the Bible are in existence. Yet the Bible has been preserved in accurate, reliable form because Biblical copyists in general, accepting the Scriptures as being divinely inspired, sought perfection in their arduous labor of producing manuscript copies of God&'s Word. Despite the care exercised by copyists of Bible manuscripts, a number of small scribal errors and alterations crept into the text. On the whole, these are insignificant and have no bearing on the Bible's general integrity. They have been detected and corrected by means of careful scholastic collation or critical comparison of the many extant manuscripts and ancient versions". Jesus, the Son of God, set the highest value on God's word, declaring, "Your word is truth.". To his followers he said: "If you remain in my word, you are really my disciples, and you will know the truth, and the truth will set you free." (John 17:17; 8:31, 32) Source "Insight on the Scriptures" published by WT Society Volume II pages 312-318