You can't just say that without explaining what you mean.
In Old Spanish, spoken from the 13th through the 15th centuries, the single sound /x/, a velar or "gutteral" fricative represented in modern Spanish by soft
g,
j, and very infrequently
x (now mostly in Mexican placenames, including
México itself) was still two different sounds: /ʃ/, a "sh" sound, represented by
x; and /ʒ/, a "zh" sound (like the
si in English
vision), represented by soft
g or
j. Though the Old Spanish period had barely ended by the time of the conquistadors, these sound values were largely still in place throughout the 1500s, and echoes of them are still found in modern Nahuatl, Catalan, and Basque, all of which use
x for /ʃ/. Over the ensuing centuries, first these two sounds merged to /ʃ/ without any spelling changing (so soft
g,
j, and
x all normally represented that "sh" sound, with
x also having its modern value of /ks/ in some words), and then shifted to /x/, the modern "gutteral" value. Spelling change came first in 1741, with the addition of a circumflex diacritic to be placed on vowels following
ch and
x to specify their pronunciation, with the circumflex designating a "classical" pronunciation, so modern
patriarca and
exactitud were spelled
patriarchâ and
exâctitud but pronounced roughly the same, while
dijo,
reloj,
ejercicio were
dixo,
relox,
exercicio; and then in 1815, which abolished "classical"
ch and the circumflex but changed all
except "classical"
x to
j, resulting in modern spellings.
Texas got its name from a Caddo word
táyshaʼ (meaning "friends, allies") during a period after the merger of the "sh" and "zh" sounds but before they shifted to their modern value, and so was originally written interchangeably with
x or
j. This suggests that a modern spelling of
Tejas would be fine, but
Texas would be just as justifiable for the same reason that
México (from Nahuatl
mēxihco, also with a "sh" sound) is normally spelt with an
x.
(source/further reading:
http://grammar.ucsd.edu/courses/lign-gs/student-materials/147 materials/sibilant-handout.pdf)