With the plethora of English versions of the Bible available today, it is remarkably simple to determine which is God's. Mark 1:2-3 indicates an undoubtable error in many of today's "Bibles".

To begin with, notice the difference in the manuscripts used by the King James translators vs. those of the NAS (as well as all modern non-KJV translations). Our beloved modern translators are in error, translating for the most part two of the Catholic manuscripts, hence leaving us to determine what is meant by the "oldest and best manuscripts".

Either the quoted material can be sourced in the unspecified but plural "prophets":

2 As <5613> it is written <1125> (5769) in <1722> the prophets <4396>, Behold <2400> (5628), I <1473> send <649> (5719) my <3450> messenger <32> before <4253> thy <4675> face <4383>, which <3739> shall prepare <2680> (5692) thy <4675> way <3598> before <1715> thee <4675>.
3 The voice <5456> of one crying <994> (5723) in <1722> the wilderness <2048>, Prepare ye <2090> (5657) the way <3598> of the Lord <2962>, make <4160> (5720) his <846> paths <5147> straight <2117> (Mar 1:2-3 KJV).

Or in the specified yet singular "Isaiah the prophet":

2 As it is written <1125> in Isaiah <2268> the prophet <4396>, "Behold <2400>, I send <649> My messenger <32a> before <4253> Your face <4383>, Who <3739> will prepare <2680> Your way <3598>;
3 The voice <5456> of one crying <994> in the wilderness <2048>, 'Make <2090> ready <2090> the way <3598> of the Lord <2962>, Make <4160> His paths <5147> straight <2117>'" (Mar 1:2-3 NAS).

Clearly, the verse quoted in Mark 1:2 is Malachi 3:1:

1 Behold, I will send <07971> (08802) my messenger <04397>, and he shall prepare <06437> (08765) the way <01870> before <06440> me (Mal 3:1 KJV):

And, nothing similar can be found in Isaiah. This solidifies the error in any version which credits Isaiah with the quote. Unfortunately, this error remains in many of the "bible" versions currently available in Christian bookstores and church pews today. If your "bible" cites Isaiah in Mark 1:2, it is in error and is therefore not the Word of God.

A correct version will state that the quote is attributable only to unspecified "prophets". This would include any of the so-called King James versions in current production today.

But, you protest, hasn't the KJV already been proven to contain many errors? For example, the dubious claim that Jehoiachin was both eight (2 Chronicles 36:9) and eighteen (2 Kings 24:8) years of age when he began to reign. A careful study of this "error" in the KJV shows that, although he was the rightful king in God's eye at age eight, he was not permitted to ascend the throne until 10 years later. The Egyptian ruler Pharaohnechoh had unlawfully delayed his promotion in 2 Kings 23:34. Chronicles presents God's viewpoint while Kings presents man's. There are hundreds of such KJV "errors", but none of them hold up to a modicum of study.

The old King James Version was God's gift to the world and has withstood the test of time ever since. Read it, and consult a dictionary for difficult or outdated words.

A secondary test is whether a version uses the pronouns thee, thou, thy, thine, and ye. As shown in my article, A Five-Minute Complete Guide to King James Grammar, these forms were obsolete even in 1611 and were included to differentiate the plural forms from the singular. Modern English is one of the few languages which uses but one form for you and your, making it impossible to discern the writer's intent as to whether he is speaking in the plural versus the singular. The NKJV completely eliminates these ancient forms, making it impossible to discern the writers' intent and can therefore be eliminated as a choice for God's Word.

The other modern versions of the KJV (the TMB, KJ21, and the DNKJV) while distingushing themselves with the inclusion or rejection of the apocrypha and the use of the name Yahweh, are so rarely found in bookstores that they can be eliminated on their scarcity.

Thus, we are left with only the KJV as a viable choice for God's Word in English.

Here's a table of popular Bible translations, ranked by approximate sales/usage and noting free versions:

Note that these figures are approximate and can vary based on the source and year. The free versions are primarily accessed through digital platforms and apps, which offer multiple translations. The King James Version is notably popular in both paid and free categories due to its public domain status and historical significance.