Mark 16:15
And He said to them, "Go into all the world and preach the gospel to all creation.

God Will Reveal Himself To Them:

Romans 1:18-20
18 For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who suppress the truth in unrighteousness, 19 because that which is known about God is evident within them; for God made it evident to them. 20 For since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen, being understood through what has been made, so that they are without excuse.

Then they have to act on it, BUT how hard is it to act on God's revelation without the Bible? It's possible but the Bible makes it easier:

Romans 1:21-32
21 For even though they knew God, they did not honor Him as God, or give thanks; but they became futile in their speculations, and their foolish heart was darkened. 22 Professing to be wise, they became fools, 23 and exchanged the glory of the incorruptible God for an image in the form of corruptible man and of birds and four-footed animals and crawling creatures. 24 Therefore God gave them over in the lusts of their hearts to impurity, that their bodies might be dishonored among them. 25 For they exchanged the truth of God for a lie, and worshiped and served the creature rather than the Creator, who is blessed forever. Amen. 26 For this reason God gave them over to degrading passions; for their women exchanged the natural function for that which is unnatural, 27 and in the same way also the men abandoned the natural function of the woman and burned in their desire toward one another, men with men committing indecent acts and receiving in their own persons the due penalty of their error. 28 And just as they did not see fit to acknowledge God any longer, God gave them over to a depraved mind, to do those things which are not proper, 29 being filled with all unrighteousness, wickedness, greed, evil; full of envy, murder, strife, deceit, malice; they are gossips, 30 slanderers, haters of God, insolent, arrogant, boastful, inventors of evil, disobedient to parents, 31 without understanding, untrustworthy, unloving, unmerciful; 32 and, although they know the ordinance of God, that those who practice such things are worthy of death, they not only do the same, but also give hearty approval to those who practice them.

Those who are not advised of the Word of God will have a hard time:

Colossians 1:9-11
9 For this reason also, since the day we heard of it, we have not ceased to pray for you and to ask that you may be filled with the knowledge of His will in all spiritual wisdom and understanding, 10 so that you will walk in a manner worthy of the Lord, to please Him in all respects, bearing fruit in every good work and increasing in the knowledge of God; 11 strengthened with all power, according to His glorious might, for the attaining of all steadfastness and patience; joyously

II Timothy 3:15-17
15 and that from childhood you have known the sacred writings which are able to give you the wisdom that leads to salvation through faith which is in Christ Jesus. 16 All Scripture is inspired by God and profitable for teaching, for reproof, for correction, for training in righteousness; 17 so that the man of God may be adequate, equipped for every good work.