So three things to keep in mind when it comes to defending the truth.
- You cannot reasonably deny the truth
- Truth is the foundation for Christianity
- Believing the truth will strengthen your faith
1 - You cannot reasonably deny the truth
If someone says, "there is no truth!" you can counter, "...but is that true, that there is no truth?" Or if you hear "we can't know what's true!", it's worth pointing out "...but is that true, that we can't know what's true?" In each case the person who tries to undermine the truth is actually making a truth statement, because it takes the truth to deny the truth. Truth cannot be reasonably denied. Even those who say "it's true for me!" assume this statement is true for you too, or else why tell you. Just as you cannot tell everyone to keep their mouth closed without opening your own, you cannot deny the truth without subtly assuming it.
2 - Truth is the foundation for Christianity
When Jesus said "I am the truth!" (John 14:6) he did not claim to say what is comfortable and convenient. In fact he was very open about the fact that choosing to follow him would be uncomfortable and inconvenient. However, Jesus claimed to be the truth and that meant everything he said corresponds to reality. Jesus tells it like it is! The Apostle Paul said truth is critical to Christianity, and it's not a case of saying, "well, it doesn't really matter if it's true because it's a good moral code - it encourages a good life!" Paul said if it is not true (and Christ is not raised from the dead) then every Christian is deluded, we're deluding others and we ought to be pitied more than anyone. Paul's point in this passage (1 Corinthians 15): Christ is risen - and Jesus appeared to many people over a long period of time to prove the point.
3 - Believing the truth will strengthen your faith
To believe you know the truth brings tremendous strength to your faith, enough to see you through the tough times. Consider John's gospel chapter 6: when Jesus fed over 5,000 people they were determined to make him King (by force if necessary), but when Jesus' teaching got harder to swallow the same people were turning around and walking away. Jesus turned to those closest to him, "Are you going to leave me too?" Step-up to the plate, Simon Peter, "Lord, where else can we go - you have the words of eternal life!" Grasp the fact that Christianity is true because Jesus Christ is the way, the truth and the life, and you will anchor your faith in an unshakeable truth.