The letter is X is indeed a mysterious letter. Per G.A. Riplinger (In Awe of Thy Word; avpublications.com), "The 24th letter of the Roman alphabet, x, is not the initial letter of any English Bible word. It does appear toward the end of Bible words" (p. 1151). This is remarkable, since it is the only letter of which this is true. Even Q and Z (the toughest letters to play in Scrabble, right?) are the first letters of at least proper names and nouns in scripture (no verbs in the Bible begin with Z). This paucity of words beginning with X also applies to a secular dictionary, compared to the number of words beginning with other letters. This brief study will consider the 13 words in the King James Bible that end in the letter x. There are, of course, others that contain the letter, but I think that it's noteworthy that 13 words end with this letter, and I offer a few thoughts later as to why.
1. box (can refer to a tree or receptacle; the receptacle is connected with the prophetic ministry and the burial of Christ, the Prophet; cf. Deut. 18:18; 2 Kings 9:1-3; Mt. 26:12, "she did it for my burial")
2. Felix (Christ-rejecting Roman governor; Acts 24:24-25)
3. flax (most references are bad; cf. Ex. 9:31, Is. 19:9; Hos. 2:5, 9)
4. flux (Paul healed Publius' father of this severe malady; Acts 28:8)
5. fox (an unclean animal; Song 2:15; Luke 13:32)
6. matrix (first appears in Ex. 13)
7. onyx (some connection with Satan; cf. Ezek. 28:13)
8. ox (some good references, but many bad ones; cf. Ex. 21:28-32; Ps. 106:20; etc.)
9. Pollux (Acts 28:11; a pagan mythological figure and sign of the boat that carries Paul to imprisonment in Rome; it was from Alexandria...in Egypt, a type of the world)
10. sardonyx (Rev. 21:20; good context--one of the foundations of new Jerusalem; it's the fifth foundation, though, and five signifies death throughout scripture, beginning in Gen. 5:5; also, sardonyx is a reddish stone, like blood...?)
11. six (how about Rom. 6:6 and Rev. 13:18--666!)
12. vex (cf. Acts 12:1 (12+1=13?); Herod, an antichrist, vexes the Jewish church, just as antichrist will in the great tribulation)
13. wax (almost always bad; cf. Mt. 24:12, 1 Tim. 5:11, etc.)
As you can see, the associations of these words are predominantly bad, though not exclusively. My persuasion, based on biblical numerology studies (my own and others'), is that the number 13 is connected with separation, good (Ex. 13:2, 12, 15--matrix!; Acts 13:2; etc.) and bad (Gen. 13:9; Acts 13:13). I also think that the letter x is connected with the cross; it actually is a cross. Where am I going with this? On the cross, our Lord Jesus Christ was separated from God, i.e. his human side. See Mt. 27:46, where he cried, "My God, my God, why hast thou forsaken me?" The Father did not forsake the Son at Calvary. Christ speaks to the Father at the beginning (Luke 23:34) and end (Luke 23:46) of his passion (Acts 1:3), showing that the Father never left the Son, though he did forsake the man Christ Jesus (1 Tim. 2:5). This had to be done, since in the garden Adam, the man, forsook God. Glorious substitution (Rom. 5:12-21)! Martin Luther said that "Every word of the Bible rings with Christ," and I would assert that every letter, including the mysterious x, points to him also, the Word of God!