1 Corinthians 15:3-4 KJV
For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
And that he was buried, and that he rose again the third day according to the scriptures:
The Apostle verified his Gospel message as being, "according to the Scriptures". Paul was not accounted an apostle until much after the death, burial and resurrection of Christ. Though I imagine he had heard of Christ and of His resurrection, he had not been an eyewitness to His life. Paul trusted the message of the Scriptures as the verification or authority for what he had not seen himself. The fact is he may have seen some of those things, but the Scriptures were even then a more authoritative reference than his own.
It is not unusual for people today to question the authority of the Bible. That is no reason to come up with other means of convincing them of truth. We can trust the power of the Gospel.
Just show them what it says and leave it there between them and God.