I do not think that Sean McDowell, the son of Josh McDowell, understands Christianity. In an appearance at The King's Academy, he said to the students, "I'm a Christian for one reason and one reason alone. ... I'm a Christian because I actually think it's true. ... If I didn't think it were true, I would give it up. I think you should only be a Christian if you actually think that it's true. That's the best reason to be a Christian."
Sorry, but that is a secondary reason as to why one is a Christian. If that is your primary reason, if that is what you are clinging to, mere intellectual head knowledge, then you are at great risk of apostatizing the faith.
The reason anyone is a Christian is for one reason and one reason alone: Christ Jesus. God the Father gave certain people to Jesus as an inheritance, and Jesus surrendered His life on their behalf, sending the Spirit to regenerate them and make them born again from above while guiding them into all truth. If we are a Christian, it is primarily because of God. Jesus makes it clear in John 6:
"No one can come to Me unless the Father who sent Me draws him" 6:44a
"For this reason I have said to you, that no one can come to Me unless it has been granted him from the Father." 6:65
Sean McDowell gave nonsensical and unbiblical advice to these students. In fact, when asked about Theistic Evolution, he did not even denounce it. God declared that everything should reproduce after its own kind. Feathers and scales have completely different DNA. Birds did not magically turn into lizards over any imagined period of time. The idea that God might use such a nonsensical concept is laughable, but it also does damage to the God of the Bible. Mr. McDowell's answer was unsatisfactory.
Mr. McDowell does not seem to understand what it means to be a Christian. "I think you should only be a Christian if you actually think that it's true. That's the best reason to be a Christian." McDowell makes it sound like a club you can decide to join at your own whim based on good information. Sorry, but that is not how Christianity works. If you merely think it is true, that is because of persuasion. If anything comes along that persuades you otherwise, you will let it go. Why? Because you were never actually a Christian to begin with; you never encountered the living God; you were never saved.
All the evidence that supports Christianity and proves it true is merely a bonus. If that is what your "Christianity" is based, then you are most likely only a religious individual and not an actual Christian. You can have none of the evidence that Christianity is true and still be a Christian, and still believe Christianity is true. It is not a matter of whether you think Christianity is true or not; it is. God does not wait for you to decide whether God exists, the Bible is true, or Christianity is true; He outright declares it to be so. Whether you believe it or not is irrelevant.
The best reason for being a Christian is knowing Jesus, having your sins forgiven, and spending eternity in Heaven. I am thankful for the knowledge I have gained and the evidence God has revealed to me that support Christianity being true, but if I had none of that I would still be a Christian. Why? Because God saved me. Out of all the possibilities He could have chosen, from people who are no doubt 100 times better than I am, God decided to save me. Not for anything I had done, but because He is a good God and He chose to love me. Had it not been for God choosing me and Jesus dying in my place, I would not be a Christian today.
I pray Sean McDowell gets this sorted out in his own life. If he does not have an actual relationship with the Lord, it does not matter how much evidence for Christianity there is, or whether or not he thinks it is true or believes it, he will find himself cast aside on judgment day.
Yes, there is plenty of evidence for the existence of Jesus, His crucifixion, and His resurrection, but that is not enough. There are people who have never seen that evidence, and yet who believe. Thomas wanted to see evidence before he would believe. What did Jesus say to him? "Blessed are those who do not see and yet believe." If you need evidence to believe, your faith is probably extremely shallow, if there at all. Christians believe because they encountered the living God and He changed them. They could not not believe even if they wanted to; they could not just "give it up."
ADDENDUM:
Regarding the question concerning the Shroud of Turin, I do not believe that it is legitimate. Why do I say this? There are constantly things popping up being claimed to be related to the crucifixion of Jesus. What do you honestly think the likelihood of finding the exact nails used in Jesus' wrists and feet? How do we know they were not re-used? Also, do you honestly believe the early Christians so superstitious as to wanting to grab the nails, the crown, Jesus' linen, etc.? I am willing to bet against that. Furthermore, these kinds of items popping up tend to be used as means of manipulating and controlling the people; trying to get people in their doors regardless of salvational status. The early Christians were more concerned with an obedient love-faith relationship with the Lord Jesus (a relationship that produced genuine kingdom fruit) than they were with anything else, let alone trinkets to later be used as religious symbols and superstitious nonsense. "Here's the actual spear that pierced Jesus' side. Ooo... Feel the power emanating from it." Have you seen how these people behave when they think they see the face of Jesus or Mary in a piece of toast?!?!?