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The intro of The X-Files famously states that "the truth is out there".
The show was created by Chris Carter. 
Chris is a nickname for Christopher.
Christopher means "Christ-bearer".

https://www.etymonline.com/word/Christopher
>  Church Latin Christophoros, from Ecclesiastical Greek khristophoros, literally "Christ-bearing;" from phoros "bearer," from pherein "to carry,"

X is a symbol for Christ.
It represents The Greek letter chi in Χριστός (Christos).
The idealist Gospel of John places an emphasis on truth.

John 8:32
https://www.drbo.org/chapter/50008.htm
>  And you shall know the truth, and the truth shall make you free.

John 14:6
https://www.drbo.org/chapter/50014.htm
>  Jesus saith to him: I am the way, and the truth, and the life. No man cometh to the Father, but by me.

John 16:13:
https://www.drbo.org/chapter/50016.htm
>  But when he, the Spirit of truth, is come, he will teach you all truth.

John 18:37-38:
https://www.drbo.org/chapter/50018.htm
>  Pilate therefore said to him: Art thou a king then? Jesus answered: Thou sayest that I am a king. For this was I born, and for this came I into the world; that I should give testimony to the truth. Every one that is of the truth, heareth my voice.
>  Pilate saith to him: What is truth? And when he said this, he went out again to the Jews, and saith to them: I find no cause in him.