Matthew 28:16-20 Now the eleven disciples went to Galilee, to the mountain to which Jesus had directed them. ¹⁷And when they saw him they worshiped him, but some doubted. ¹⁸And Jesus came and said to them, “All authority in heaven and on earth has been given to me. ¹⁹Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, ²⁰teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age.”

Isaiah 6:1-8 (ESV) In the year that King Uzziah died I saw the Lord sitting upon a throne, high and lifted up; and the train of his robe filled the temple. ²Above him stood the seraphim. Each had six wings: with two he covered his face, and with two he covered his feet, and with two he flew. ³And one called to another and said: “Holy, holy, holy is the Lord of hosts; the whole earth is full of his glory!” ⁴And the foundations of the thresholds shook at the voice of him who called, and the house was filled with smoke. ⁵And I said: “Woe is me! For I am lost; for I am a man of unclean lips, and I dwell in the midst of a people of unclean lips; for my eyes have seen the King, the Lord of hosts!” ⁶Then one of the seraphim flew to me, having in his hand a burning coal that he had taken with tongs from the altar. ⁷And he touched my mouth and said: “Behold, this has touched your lips; your guilt is taken away, and your sin atoned for.” ⁸And I heard the voice of the Lord saying, “Whom shall I send, and who will go for us?” Then I said, “Here I am! Send me.”

The Athanasian Creed
Whoever wishes to be saved must, above all else, hold to the true Christian faith. Whoever does not keep this faith pure in all points will certainly perish forever.

Now this is the true Christian faith: We worship one God in three persons and three persons in one God, without mixing the persons or dividing the divine being. For each person—the Father, the Son, and the Holy Spirit—is distinct, but the deity of Father, Son, and Holy Spirit is one, equal in glory and coeternal in majesty. What the Father is, so is the Son, and so is the Holy Spirit. The Father is uncreated, the Son uncreated, the Holy Spirit uncreated; the Father is infinite, the Son infinite, the Holy Spirit infinite; the Father is eternal, the Son eternal, the Holy Spirit eternal; yet they are not three who are eternal, but there is one who is eternal, just as they are not three who are uncreated, nor three who are infinite, but there is one who is uncreated and one who is infinite. In the same way the Father is almighty, the Son is almighty, and the Holy Spirit is almighty; yet they are not three who are almighty, but there is one who is almighty. So the Father is God, the Son is God, the Holy Spirit is God; yet they are not three Gods, but one God. So the Father is Lord, the Son is Lord, the Holy Spirit is Lord; yet they are not three Lords, but one Lord. For just as Christian truth compels us to confess each person individually to be God and Lord, so the true Christian faith forbids us to speak of three Gods or three Lords. The Father is neither made nor created nor begotten of anyone. The Son is neither made nor created, but is begotten of the Father alone. The Holy Spirit is neither made nor created nor begotten, but proceeds from the Father and the Son. So there is one Father, not three Fathers; one Son, not three Sons; one Holy Spirit, not three Holy Spirits. And within this Trinity none comes before or after; none is greater or inferior, but all three persons are coequal and coeternal, so that in every way, as stated before, all three persons are to be worshiped as one God and one God worshiped as three persons. Whoever wishes to be saved must have this conviction of the Trinity.

It is furthermore necessary for eternal salvation truly to believe that our Lord Jesus Christ also took on human flesh. Now this is the true Christian faith: We believe and confess that our Lord Jesus Christ, God’s Son, is both God and man. He is God, eternally begotten from the nature of the Father, and he is man, born in time from the nature of his mother, fully God, fully man, with rational soul and human flesh, equal to the Father as to his deity, less than the Father as to his humanity; and though he is both God and Man, Christ is not two persons but one, one, not by changing the deity into flesh, but by taking the humanity into God; one, indeed, not by mixture of the natures, but by unity in one person; for just as the rational soul and flesh are one human being, so God and man are one Christ. He suffered for our salvation, descended into hell, rose the third day from the dead. He ascended into heaven, is seated at the right hand of God the Father almighty, and from there he will come to judge the living and the dead. At his coming all people will rise with their own bodies to answer for their personal deeds. Those who have done good will enter eternal life, but those who have done evil will go into eternal fire.

This is the true Christian faith. Whoever does not faithfully and firmly believe this cannot be saved.