How can I know the truths of Christianity are true?
The defense of Christianity is based on the defense of who Jesus was and what he did. This cannot come by means of science because it can’t be observed and tested. You have to search for the truth by way of history. Are the claims of Jesus historically accurate and true? And that depends on the reliability of the historian. Luke wrote the gospel that bears his name and the book of Acts. He was detailed and factual in all his writings. Read what Luke himself tells us of his methods.
Many have undertaken to draw up an account of the things that have been fulfilled among us, just as they were handed down to us by those who from the first were eyewitnesses and servants of the word. Therefore, since I myself have carefully investigated everything from the beginning, it seemed good also to me to write an orderly account for you, most excellent Theophilus.
What Jesus did, He did in public for all to see. He did not get some revelation from God in a secret cave or go to some secluded mountaintop to attain the wisdom of the universe. Everything that Jesus taught, the miracles He performed, was done for all to see and refute if it was not true. Jesus claimed to be God and to prove that He was, raised the dead, healed the blind and made the lame walk again.
The other gospel writers, Matthew, Mark and John also wrote of Jesus’ life and even though they emphasize different details of His they life, confirm the truthfulness, historically speaking, of what Jesus did and who Jesus claimed to be. Jesus also said that He would die on a cross and rise from the dead in three days, and that is what history confirms.
Seeing what was ahead, he (Peter) spoke of the resurrection of the Christ, that He was not abandoned to the grave, nor did His body see decay. God has raised this Jesus to life, and we are all witnesses of the fact.
As we can trust the true history that there was an Alexander the Great who conquered much of the known world in his day and that Henry VIII became king of England in 1509, we can also trust the true history of who Jesus Christ was and what he did.