Immediately that I set myself to think upon whatever is true I ran into the same question as Pilate put to Jesus, “What is truth?” (John 18:38). That is a profound and troublesome question, but don’t worry, I’m not going to delve into the philosophical notions of knowledge.
Realistically though, how do we know if something is true? Jesus, in his prayer in John 17:6-19 reveals that through being given the words of God by Jesus his disciples have come to know truth.
What truth had they come to know? In the context of chapters 14 to 17 of John’s gospel, it is clear that Jesus himself is the truth (John 14:6).
So, in Philippians 4:8 Paul is exhorting us to think about Jesus as the truth. The only sure way we can think rightly about Jesus is by what is revealed of him in the Bible. This has the added (or inherent) benefit of increasing our sanctification, our freedom from sin (John 17:17). By remaining faithful to the words of God, those of us who believe in Jesus will know the truth and he will set us free (John 8:31-32).