It is the continuation of the thought, therefore, a colon is appropriate and not a period.
Yes. A colon (continuation) links the two verses together more closely than a period (hard stop) or semi-colon (softer stop and closer link than a period). They are literary tools for writers and translators to convey thoughts and ideas.
A colon
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
) is pretty much synonymous with a dash (-) where both punctuation marks are a "continuation of the thought" as you noted. They can also denote a definition where the sentence that comes after a colon or dash defines and/or elaborates the preceding sentence.
The NIV translators chose to use a period instead which is more appropriate today as
@Nehemiah6 noted:
Hebrews 12:21-22
New International Version
20 because they could not bear what was commanded: “If even an animal touches the mountain, it must be stoned to death.”[
a] 21 The sight was so terrifying that Moses said, “I am trembling with fear.”[
b]
21 The sight was so terrifying that Moses said, “I am trembling with
fear.”
22 But you have come to Mount Zion, to the city of the living God, the heavenly Jerusalem. You have come to thousands upon thousands of angels in joyful assembly,
Interestingly, the NKJV translators made this correction as well but left in the brackets that start at verse 20:
Hebrews 12:21-22
New King James Version
20 (For they could not endure what was commanded: “And if so much as a beast touches the mountain, it shall be stoned [
a]or shot with an arrow.”
21 And so terrifying was the sight
that Moses said, “I am exceedingly afraid and
trembling.”)
22 But you have come to Mount Zion and to the city of the living God, the heavenly Jerusalem, to an innumerable company of angels,
Here are the original KJV verses for comparison:
Hebrews 12:20-22
King James Version
20 (For they could not endure that which was commanded, And if so much as a beast touch the mountain, it shall be stoned, or thrust through with a dart:
21 And so terrible was the sight, that Moses said, I exceedingly fear and
quake![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
22 But ye are come unto mount Sion, and unto the city of the living God, the heavenly Jerusalem, and to an innumerable company of angels,
I like the different words chosen for each translation as well (quake, trembling, and fear) that shows how language evolves over time.