Introduction:
After we become Christians, not only do we often feel the urge to share the "unexplainable joy" (1 Pet 1:8) with others, we are actually commanded to share the Gospel [evangelize] (2 Cor 5:18-20, Rom 1:16, Philippians 1:7,17, 1 Peter 3:15).
Of course the Bible is where God's Nature and Plan is revealed but we would limit the potential if we didn't realize and acknowledge something God has done in humanity.
God has put into all humanity what might be called an innate or obvious awareness of God. Simply, it seems every human "feels" there is some sort of god, even if that is as far as it goes.
Psalm 14:1/Psalm 53:1
The fool has said in his heart, “There is no God.” They are corrupt, They have done abominable works, There is none who does good.
Romans 1:20
For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they [who would deny God exists] are without excuse,
Paul, the apostle when interacting with non-believer (Greeks) even starts with the fact that these people at least believed in some sort of God.
Acts 17:22-29
Then Paul stood in the midst of the Areopagus [debate forum] and said, “Men of Athens, I perceive that in all things you are very religious; for as I was passing through and considering the objects of your worship, I even found an altar with this inscription: TO THE UNKNOWN GOD. Therefore, the One whom you worship without knowing, Him I proclaim to you: God, who made the world and everything in it, since He is Lord of heaven and earth, does not dwell in temples made with hands. Nor is He worshiped with men’s hands, as though He needed anything, since He gives to all life, breath, and all things. And He has made from one blood every nation of men to dwell on all the face of the earth, and has determined their preappointed times and the boundaries of their dwellings, so that they should seek the Lord, in the hope that they might grope for Him and find Him, though He is not far from each one of us; for in Him we live and move and have our being, as also some of your own poets have said, ‘For we are also His offspring.’Therefore, since we are the offspring of God, we ought not to think that the Divine Nature is like gold or silver or stone, something shaped by art and man’s devising.
John Calvin commented on Romans 1:20 by saying this:
“…he [Paul] seems here to have intended to indicate a manifestation, by which they might be so closely pressed, that they could not evade; for every one of us undoubtedly finds it to be engraven on his own heart, By saying, that God has made it manifest, he means, that man was created to be a spectator of this formed world, and that eyes were given him, that he might, by looking on so beautiful a picture, be led up to the Author himself…God is in himself invisible; but as his majesty shines forth in his works and in his creatures everywhere, men ought in these to acknowledge him, for they clearly set forth their Maker: and for this reason the Apostle in his Epistle to the Hebrews says, that this world is a mirror, or the representation of invisible things. He does not mention all the particulars which may be thought to belong to God; but he states, that we can arrive at the knowledge of his eternal power and divinity…He plainly testifies here, that God has presented to the minds of all the means of knowing him, having so manifested himself by his works, that they must necessarily see what of themselves they seek not to know — that there is some God; for the world does not by chance exist, nor could it have proceeded from itself. We conceive that there is a Deity; and then we conclude, that whoever he may be, he ought to be worshipped: but our reason here fails, because it cannot ascertain who or what sort of being God is. But this knowledge of God, which avails only to take away excuse, differs greatly from that which brings salvation. He plainly testifies here, that God has presented to the minds of all the means of knowing him, having so manifested himself by his works, that they must necessarily see what of themselves they seek not to know — that there is some God; for the world does not by chance exist, nor could it have proceeded from itself.” – source
The point is, whether in defending our Christianity or evangelizing to others, a good and demonstrated starting point is every persons innate awareness of god. Whether the person has any clue about what or who is god (as the Greeks called him the "unknown god"), we can start there and then speak of the Bible as the one and only God's account of Himself. However, if a person won't acknowledge this basic point, then they are as a fool, and not just as a dumb person but as a person with mental limitations that will make it impossible to go any further.
