And that is a great point. Sometimes the open door comes as a result of a question- Why do a lot of Christians believe in a Rapture? or Noah's Ark or a young earth or the Shroud? Controversy! 1 Peter 3:15 says this: But in your hearts revere Christ as Lord. Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have. But do this with gentleness and respect,
As I touched on above, Paul used that altar to an unknown god, then Paul actually quotes a poem (I looked it up, it's from the classical Greek- the poet Epimenides) "in Him we live and move and have our being". and then "For we are also His offspring"
My point is that knowledge and understanding of background cultural stuff are useful at times. Paul uses these as a conversational opener. He doesn't stay there, but goes on to preach the Word of God, the gospel. And some in Athens believed.
Were those things of God? Were they true? Well they were there at the time, and Paul used them to strike up a conversation. This altar sat there with that inscription- if we looked at it, we would say that altar was likely pagan. But Paul used it. Epimenides wasn't saved- he was a pagan, a Greek and the poem was about Zeus. And Paul stuck it in there.
Whether the Shroud (Sideshow) is real or not, it can open doors. If you have that conversation arise, just move on quickly like Paul does to the actual teaching of the gospel. You can briefly explain why you think it's not the real Shroud using the Bible verses to show what you believe. Then move on to whether this person believes that Jesus did exist, and who He claimed to be.
Here is the passage: Acts 17: 22-34
22 Then Paul stood in the midst of the Areopagus and said, “Men of Athens, I perceive that in all things you are very religious;
23 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:
24 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.
25 Nor is He worshiped with men’s hands, as though He needed anything, since He gives to all life, breath, and all things.
26 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,
27 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;
28 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.’
29 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.
30 Truly, these times of ignorance God overlooked, but now commands all men everywhere to repent,
31 because He has appointed a day on which He will judge the world in righteousness by the Man whom He has ordained. He has given assurance of this to all by raising Him from the dead.”
32 And when they heard of the resurrection of the dead, some mocked, while others said, “We will hear you again on this matter.”
33 So Paul departed from among them.
34 However, some men joined him and believed, among them Dionysius the Areopagite, a woman named Damaris, and others with them.
