Given the overwhelming magnitude, unlimited power and inexpressible wonder of God, not only could no human being see Him face to face and live no one could experience Him and fail to submit to Him. For this reason, although He has made the truth of His existence and His perfect character plain for all mankind to see, God has also integrated "deniability" into human history. That is to say, God has given us all the possibility, potential and ability to live our lives as if He did not exist, and has even given us the capability of hardening our hearts to point of denying that He exists at all. The heavens proclaim His glory (Ps.19:1), but we are free to "tune out" their message and are easily able to do so. It is only under such conditions that the true choice of every man and woman could be made in genuine freedom without undue coercion or duress. This world is God's smelter and it puts our authentic free will to the test, demonstrating what is silver and gold, and what is merely dross. The truth is veiled, but it is not inaccessible; it is possible to ignore, but it is available for all who truly want it.
Acts 17:26-27 (NIV)
26 From one man he made all the nations, that they should inhabit the whole earth; and he marked out their appointed times in history and the boundaries of their lands. 27 God did this so that they would seek him and perhaps reach out for him and find him, though he is not far from any one of us.
For all who are even the least bit interested in making the smallest effort to know about Him, God provides the living water of life, the gospel of truth; and as the verse above indicates, God has designed every aspect of human life in the case of every single human being who has ever lived with the express purpose of offering salvation to any and all who are willing to accept it.
John 6:44-45 NASB
"No one can come to Me unless the Father who sent Me draws him; and I will raise him up on the last day. It is written in the prophets, 'And they shall all be taught of God.' Everyone who has heard and learned from the Father, comes to Me."
God has designed the universe, the human spirit, and the process of history in such a way so that no one who stands before Him at the last judgment will be able to say "I never had any idea of You or my mortality or my need for salvation". However, it is pointless to "draw" to Jesus those who are unwilling to accept Him if so drawn (one of our Lord's main points here in John chapter six as He remonstrates with those who saw the miracle of the bread and fish but would not accept Him as the Bread of Life). Only those who are willing are benefitted by being "drawn", that is, by being led to a hearing of the truth, that is, the gospel. The individuals in question in these verses have "heard" but they have not "learned". The Greek verb here is cognate with the word "disciple" and means "learned" in the sense of accepting the truth so as to make it one's own in the manner of a disciple. That is to say, this phrase indicates more than mere intellectual understanding; it indicates an acceptance of the truth heard as true and a willingness to respond to it. Jesus is speaking here of the faith that follows perception of the gospel and leads to salvation (i.e., "coming to Me" = "[having] learned" = responding to the gospel in faith). Hearing the truth, knowing the truth is not enough; in order to be saved, a person has to submit, yield, believe, accept, trust . . . use his/her free will in response to the Lord and to the truth that Jesus is the One who died for our sins and that we are delivered from death through the work of the Son of God. The human condition as God has ordained it in His infinite wisdom guarantees us the knowledge of the truth, but also the ability to ignore and even deny the truth: only those who have both "heard" and "learned" (i.e., have actually accepted the need for salvation so as to respond to it) come to be saved.
Some have twisted the fact of the veiling of God's glory and the sometimes seemingly limited availability of the gospel into an excuse as if anything less than complete and universal revelation relieves mankind of all accountability. In fact, of course, all of us are accountable to God, and would be even if He were not offering the truth of the gospel to all and even if He had not sacrificed His One and only beloved Son on our behalf so that we might be saved. The reality, however, is that God's truth is universally available for all who desire it and, indeed, is universally undeniable at a certain basic level to everyone (at least initially). Inherent in the nature of the universe, the nature of the human spirit, and the nature of human life are God's most basic truths, made undeniable through the process known in theology as "natural revelation".