A voice cries: “In the wilderness prepare the way of the LORD; make straight in the desert a highway for our God. -- Isaiah 40:3 (ESV)

Which was quoted by John the Baptist:

In those days John the Baptist came preaching in the wilderness of Judea, “Repent, for the kingdom of heaven is at hand.”

For this is he who was spoken of by the prophet Isaiah when he said,

“The voice of one crying in the wilderness: ‘Prepare the way of the Lord; make his paths straight.’”

...

Then Jesus came from Galilee to the Jordan to John, to be baptized by him.

-- Matthew 3:1-3,13 (ESV)

Some assume that John was preparing the way of Jesus which would mean that Jesus would be "the LORD" and "our God" as prophesied by Isaiah, but if you continue reading Isaiah, the next verse states:

• Every valley shall be lifted up, and
• every mountain and hill be made low;
• the uneven ground shall become level, and
• the rough places a plain.
• And the glory of the LORD shall be revealed, and
• all flesh shall see it together,

for the mouth of the LORD has spoken.

-- Isaiah 40:4 (ESV)

None of these happened when Jesus arrived at John's baptism.

First message: Repent and "bear good fruit"

"The voice in the wilderness" was crying that people need to repent because of God's "wrath" that would come and not because Jesus will come to be baptised. Even Matthew made this point clear when he recorded that John's main concern was that people needed to repent:

But when he saw many of the Pharisees and Sadducees coming to his baptism, he said to them,

“You brood of vipers! Who warned you to flee from the wrath to come?
Bear fruit in keeping with repentance.
And do not presume to say to yourselves, ‘We have Abraham as our father,’ for I tell you, God is able from these stones to raise up children for Abraham. Even now the axe is laid to the root of the trees. Every tree therefore that does not bear good fruit is cut down and thrown into the fire.

-- Matthew 3:7-10 (ESV)

This was John's main concern with that specific prophecy. This is also evident in the parallel passages (Luke 3:7-14 and Mark 1:4-5) when John's main concern is repentance and that the people's repentance should show "bear good fruit".

Second message: John is not the Christ

People confused John with the prophesied Christ. That is why Luke wrote:

As the people were in expectation, and all were questioning in their hearts concerning John, whether he might be the Christ, John answered them all, saying,

“I baptize you with water, but he who is mightier than I is coming, the strap of whose sandals I am not worthy to untie. He will baptize you with the Holy Spirit and fire.” -- Luke 3:15-16 (ESV)

John made it clear how he is different from the Christ.

John never said that Jesus is God, instead he only said that Jesus is "mightier" and more "worthy" because Jesus can baptize with "the Holy Spirit" and "fire".

These are two different points that John made which should not be confused with each other.

Jesus prophetic fulfillment

As they went away, Jesus began to speak to the crowds concerning John:

“What did you go out into the wilderness to see?
A reed shaken by the wind?
What then did you go out to see?
A man dressed in soft clothing?

Behold, those who wear soft clothing are in kings’ houses.

What then did you go out to see?
A prophet?

Yes, I tell you, and more than a prophet. This is he of whom it is written,

“ ‘Behold, I send My messenger before your face, who will prepare your way before you.’

Truly, I say to you, among those born of women there has arisen no one greater than John the Baptist.

-- Matthew 11:7-11 (ESV)

Jesus quoted Malachi:

You have wearied the LORD with your words.

But you say, “How have we wearied him?” By saying, “Everyone who does evil is good in the sight of the Lord, and he delights in them.” Or by asking, “Where is the God of justice?”

"Behold, I send My messenger, and he will prepare the way before Me. And the Lord whom you seek will suddenly come to his temple; and the messenger of the covenant in whom you delight, behold, he is coming", says the LORD of hosts.

-- Malachi 2:17-3:1 (ESV)

It is unfortunately that the translators of Malachi choose to replace YHVH's name with the LORD, because this cause confusion with the other Lord.

This prophecy is about a "messenger", who will prepare the people for YHVH's (the LORD's) "justice" (Malachi 2:17).

Then Malachi also mentions a second "Lord whom will come to his temple" which:

• is not capitalized, and
• unlike YHVH (the first LORD) who was speaking in first person, this Lord is referred as a second person.

The second Lord is likely a reference to Jesus, which should not be confused with YHVH (the LORD).