Only Begotten — the Only Son of the Father
Firstborn means born first. That is, no one else has been born the way the Son of God was born
“And the Word became flesh and dwelt among us, and we beheld His glory, the glory as of the only begotten of the Father, full of grace and truth” (John. 1:14)
“No one has seen God at any time. The only begotten Son, who is in the bosom of the Father, He has declared Him” (John. 1:18)
“For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life” (John. 3:16)
“He who believes in Him is not condemned; but he who does not believe is condemned already, because he has not believed in the name of the only begotten Son of God” (John. 3:18)
“But when the fullness of the time had come, God sent forth His Son, born of a woman, born under the law” (Gal. 4:4)
“In this the love of God was manifested toward us, that God has sent His only begotten Son into the world, that we might live through Him” (1 John. 4:9)
Abraham’s sacrifice of his only begotten son is a prefiguration of the sacrifice of the only begotten Son of God on the cross:
“By faith Abraham, when he was tested, offered up Isaac, and he who had received the promises offered up his only begotten son” (Heb. 11:17)
Jesus Christ — the Firstborn
Christ is called the firstborn:
“For whom He foreknew, He also predestined to be conformed to the image of His Son, that He might be the firstborn among many brethren” (Rom. 8:29)
“But when He again brings the firstborn into the world, He says: ‘Let all the angels of God worship Him’” (Heb. 1:6)
Why is Christ the firstborn? Because He was the first to rise from the dead:
“And He is the head of the body, the church, who is the beginning, the firstborn from the dead, that in all things He may have the preeminence” (Col. 1:18)
“and from Jesus Christ, the faithful witness, the firstborn from the dead, and the ruler over the kings of the earth. To Him who loved us and washed us from our sins in His own blood” (Rev. 1:5)
“that the Christ would suffer, that He would be the first to rise from the dead, and would proclaim light to the Jewish people and to the Gentiles” (Act. 26:23)
By the Way
According to Islam, Allah neither begot nor was begotten:
“He (Allah) begets not, nor was He begotten” (Quran, Surah 112, Ayah 3)