This is the one who was mentioned by the prophet Isaiah when he said,

“The voice of one shouting in the wilderness,
‘Prepare the road for the Lord,
Make His highways straight (level, direct)!’”

This was to fulfill what was spoken by the prophet Isaiah:

so that He fulfilled what was spoken by the prophet Isaiah: “He Himself took our infirmities [upon Himself] and carried away our diseases.”

This was to fulfill what was spoken by the prophet Isaiah:

In them the prophecy of Isaiah is being fulfilled, which says,

‘You will hear and keep on hearing, but never understand;
And you will look and keep on looking, but never comprehend;

You hypocrites (play-actors, pretenders), rightly did Isaiah prophesy of you when he said,

As it is written and forever remains in the [writings of the] prophet Isaiah:

“Behold, I send My messenger ahead of You,
Who will prepare Your way—

He replied, “Rightly did Isaiah prophesy about you hypocrites (play-actors, pretenders), as it is written [in Scripture],

‘These people honor Me with their lips,
But their heart is far from Me.

as it is written and forever remains written in the book of the words of Isaiah the prophet,

“The voice of one shouting in the wilderness,
‘Prepare the way of the Lord,
Make His paths straight.

The scroll of the prophet Isaiah was handed to Him. He unrolled the scroll and found the place where it was written,

He said, “I am the voice of one shouting in the wilderness, ‘Make straight the way of the Lord,’ as the prophet Isaiah said.”

This was to fulfill what Isaiah the prophet said: “Lord, who has believed our message? And to whom has the arm (the power) of the Lord been shown (unveiled, revealed)?”

Therefore they could not believe, for Isaiah said again,

Isaiah said these things because he saw His glory and spoke about Him.

and he was returning, and sitting in his chariot he was reading [the scroll of] the prophet Isaiah.

Philip ran up and heard the man reading the prophet Isaiah, and asked, “Do you understand what you are reading?”

They disagreed among themselves and they began to leave after Paul had made one last statement: “The Holy Spirit rightly spoke through Isaiah the prophet to your fathers, saying,

And Isaiah calls out concerning Israel: “Though the number of the sons of Israel be like the sand of the sea, it is only the remnant [a small believing minority] that will be saved [from God’s judgment];

It is as Isaiah foretold,

“If the Lord of Hosts had not left us seed [future generations from which a believing remnant of Israelites came],
We would have become like Sodom, and would have resembled Gomorrah [totally rejected and destroyed]!”

But they did not all pay attention to the good news [of salvation]; for Isaiah says, “Lord, who has believed our report?”

Then Isaiah is very bold and says,

“I have been found by those who did not seek Me;
I have shown Myself to those who did not [consciously] ask for Me.”

Again Isaiah says,

“There shall be a root of Jesse,
He who arises to rule [as King] over the Gentiles,
In Him shall the Gentiles hope.”