Jesus Christ is the Only-Begotten Son of God and Our Lord and God
First, we are going to show, on the authority of God -- the true Author of the Sacred Scripture -- and on the authority of His divine institution -- His Mystical Body, the infallible Holy Roman Catholic Church, which approved the Sacred Scripture -- that Jesus Christ is the Only-Begotten Son of God, and that He is Our Lord and God.
Matthew 16:15-17. "Jesus saith to them: But whom do you say that I am? Simon Peter answered and said: Thou art Christ, the Son of the living God. And Jesus answering said to him: Blessed art thou, Simon Bar-Jona: because flesh and blood hath not revealed it to thee, but my Father who is in heaven."
Matthew 26:63-64. "But Jesus held his peace. And the high priest said to him: I adjure thee by the living God, that thou tell us if thou be the Christ the Son of God. Jesus saith to him: Thou hast said it. Nevertheless I say to you, hereafter you shall see the Son of man sitting on the right hand of the power of God and coming in the clouds of heaven."
Luke 22:66-70. "And as soon as it was day, the ancients of the people and the chief priests and scribes came together. And they brought him into their council saying: If thou be the Christ, tell us. And he saith to them: If I shall tell you, you will not believe me. And if I shall also ask you, you will not answer me, nor let me go. But hereafter the Son of man shall be sitting on the right hand of the power of God. Then said they all: Art thou then the Son of God? And he said: You say that I am."
After the resurrection, Jesus visited the Apostles.
Matthew 20:24-29. "Now Thomas, one of the twelve, who is called Didymus, was not with them when Jesus came. The other disciples therefore said to him: We have seen the Lord. But he said to them: Except I shall see in his hands the print of the nails and put my finger into the place of the nails and put my hand into his side, I will not believe. And after eight days, again his disciples were within, and Thomas with them. Jesus cometh, the doors being shut, and stood in the midst and said: Peace be to you. Then he said to Thomas: Put in thy finger hither and see my hands. And bring hither the hand and put it into my side. And be not faithless, but believing. Thomas answered and said to him: My Lord and my God. Jesus saith to him: Because thou hast seen me, Thomas, thou hast believed: blessed are they that have not seen and have believed."
We believe in one God the Father almighty, creator of all things visible and invisible. And in our one Lord Jesus Christ the Son of God, the only-begotten born of the Father, that is of the substance of the Father, God of God, light of light, true God of true God, born, not made, of one substance with the Father (which they call in Greek "homousion"), by whom all things were made, which are in heaven and on earth, who for our salvation came down, and became incarnate and was made man, and suffered, and arose again on the third day, and ascended into heaven, and will come to judge the living and the dead. And in the Holy Spirit.
Pope St. Leo IX on his epistle "Congratulamur vehementer" to Peter, Bishop of Antioch, April 13, 1053 declared the symbol of faith:
For I firmly believe that the Holy Trinity, the Father and the Son and the Holy Spirit, is one omnipotent God, and in the Trinity the whole Godhead is co-essential and consubstantial, co-eternal and co-omnipotent, all things, through whom all things, in whom all things [Rom. 11:36] which are in heaven or on earth, visible or invisible. Likewise I believe that each person in the Holy Trinity is the one true God, complete and perfect.
I believe also that the Son of God the Father, the Word of God, was born eternallly before all time from the Father, consubstantial, co-omnipotent, and co-equal to the Father through all things in divinity; born of the Holy Spirit from the ever virgin Mary in time, with a rational soul, having two nativities, the one from the Father, eternal, the other from the Mother, in time; having two wills and opcrations, true God and true man, individual in each nature and perfect, not having suffered a fusion and division, not adopted or phantastical, the one and onlv God, the Son of God in two natures, but in the singleness of one person, incapable of suffering and immortal in divinity; but in humanity for us and for our salvation suffered in the true passion of the body and was buried, and arose from the dead on the third day in the true resurrection of the body; because of which we must declare with the disciples that He ate from no need of food but only from will and power; on the fortieth day after His resurrection with the flesh in which He arose, and with His soul He ascended into heaven and sits at the right hand of the Father, whence on the tenth day He sent the Holy Spirit, and thence, as He ascended, He will come to judge the
living and the dead, and will render to each one according to his works.
I believe also that the Holy Spirit, complete and perfect and true God, proceeding from the Father and the Son, co-equal, co-essential, co-omnipotent and coeternal with the Father and the Son in all respects, has spoken through the prophets.
That this holy and individual Trinity is not three Gods, but in three persons and in one nature and or essence [is] one God omnipotent, external, invisible and incommutable, so I believe and confess, so that I may truly proclaim that the Father is not begotten, the Son is the only begotten one, and the Holy Spirit is neither begotten nor unbegotten, but proceeds from the Father and the Son. I believe that the one true Church is holy, Catholic and apostolic, in which is given one baptism and the true remission of all sins. I also believe in a true resurrection of this body, which now I bear, and in eternal life.
I believe also that there is one author of the New and Old Testament of the law both of the Prophets and of the Apostles, namely the omnipotent God and Lord. (I believe) that God predestined only the good things, but that He foreknew the good and the evil. I believe and profess that the grace of God precedes and follows man, yet in such a manner that I do not deny free will to the rational creature. I also believe and declare that the soul is not a part of God but was created from nothing and was without baptism subject to original sin.
Furthermore, I declare anathema every heresy raising itself against the holy Catholic Church, and likewise him whosoever has honored or believes that any writings beyond those which the Catholic Church accepts ought to be held in authority or has venerated them. I accept entirely the four Councils and I venerate them as the four Gospels, because through four parts of the world the universal Church, upon these as on square stone, has been founded . . . . Equally I accept and venerate the three remaining Councils. . . . Whatever the above mentioned seven holy and universal Councils believe and praise I also believe and praise, and whomever they declare anathema, I declare anathema. (Denzinger, The Sources of Catholic Dogma, 30th Edition, # 343-349, p. 141-142)
But it is not enough to believe in Jesus Christ to be saved. To be saved, one has belong to the Holy Roman Catholic Church, outside of which there is no salvation. Various Popes in the past has defined ex-cathedra the dogma "Outside the Roman Catholic Church there is no salvation" (extra Ecclesiam nullam esse salutem) and has condemned the error of indifferentism which says that all religions can save.
Pope Boniface VIII in the Bull "Unam Sanctam", November 18, 1302, declared ex-cathedra:
"We firmly believe and simply confess this (Church) outside which there is no salvation nor remission of sin." (Denzinger, The Sources of Catholic Dogma, 30th Edition, # 468, p. 186) (Note: the Bull is the most authoritative document of the Church which the Pope signs and seals with his ring [bulla].)
"Furthermore, we declare, say, define, and proclaim to every human creature that they by necessity for salvation are entirely subject to the Roman Pontiff." (Denzinger, ibid. , 30th Edition, # 469, p. 187)
Pope Eugene IV declared ex-cathedra in the Bull "Cantata Domino" on February 4, 1442: