John 20:31 says that to be saved, we should believe Jesus is the Christ and Son of God. Rom. 10:9 says to be saved, we should believe he is Lord. But I don't see any verse saying that to be saved, we need to believe he is God, so I think it is optional. Yet many Christians believe we also have to believe he is God. Are they adding salvation requirements? What is your position?
First, you are misreading John 20:31. It does not say “believe Jesus is the Christ and Son of God” in the sense that John is referring to two different persons. What John writes is “Jesus is the Christ, the Son of God.” The phrase is appositional. Jesus is the historical person; The Son of God describes his deity. This comports with Matthew 1:23 where, speaking of the birth of Jesus, his name is given as “Emmanuel,” meaning “God with us.” Clearly the scripture teaches that Jesus is God. Read John 1:1, 14. The Word was God and the Word came and dwelled among us. Since the scripture teaches that Jesus is God, to disbelieve that truth is to disbelieve the scripture. In the same context that you cite, Thomas refers to Jesus as “My Lord and My God.” John 20:28. Jesus neither rebuked Thomas nor did he deny the truthfulness of the confession.
Second, Jesus stated or strongly implied that he is God in such passages as Matt. 11:27; Mark 2:5, 10; 12:37; Luke 10:22; John 16:14-15; 14:26; 15:26; 16:7.
Third, his miracles declare Christ to be God. John 20:30-31.
Fourth, his resurrection, prophesied and performed, declare him to be God.
Fifth, scripture declares the fullness of the Godhead dwelled in Christ. Colossians 2:9.
Sixth, the very words that you use – “Son of God” and “Lord” – declare that Jesus is God. The Christological titles are generally considered to be Son of man, Son of God, Christ, and the Lord. The titles Son of David, Son of God, and Messiah were used of Jesus by others. Son of Man was the title most often applied to Jesus by himself. In Mark 14:62, Jesus acknowledged that the title applied to him in the context of a direct claim to be the Messiah. “Christ” means the anointed one or Messiah.
Seventh, Paul, in describing the preexistence of Jesus, declared him to be in the form of God and equal with God. Philippians 2:5-11. Jesus claimed to be preexistent and that he was the “I Am,” (John 8:48; John 17:5). In doing so he used the title that belonged to God. (Exodus 3:14.)
With these passages in mind, go to 1 John 2:18, 22; 1 John 4:3; and 2 John 1:7. These passages speak of and identify the antichrist. 1 John 2:18 declares his coming and that many were even then present; 1 John 2:22 declares the one who denies that Jesus is the Christ (Messiah) is the antichrist and denies the Son, and that he who denies the Son does not have the Father. I John 4:3 identifies at least one of the specifics that the antichrist denies – that Jesus Christ is come in the flesh. Remember that it was the Word that came in the flesh, John 1:14, and that the Word was God. John 1:1. Thus that which the antichrist denied was that Christ is God. This truth is repeated in 2 John 1:7.
Much more could be written on this subject. Indeed, entire books have been written on the deity of Christ. However, this will suffice for the purpose here – to demonstrate that Jesus is God, belief that he is God is not optional, and failure to believe that Jesus Christ is God will condemn the soul of the unbeliever. “23 And he said unto them, Ye are from beneath; I am from above: ye are of this world; I am not of this world. 24 I said therefore unto you, that ye shall die in your sins: for if ye believe not that I am he, ye shall die in your sins. 25 Then said they unto him, Who art thou? And Jesus saith unto them, Even the same that I said unto you from the beginning.” John 8:23-25.
You Must Hear the Gospel
Romans 10:17
You must hear the gospel and then understand and recognize that you are lost without Jesus Christ no matter who you are and no matter what your background is. The Bible tells us that “all have sinned, and come short of the glory of God.” (Romans 3:23) Before you can be saved, you must understand that you are lost and that the only way to be saved is by obedience to the gospel of Jesus Christ. (2 Thessalonians 1:8) Jesus said, “I am the way, the truth, and the life: no man cometh unto the Father, but by me.” (John 14:6) “Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved.” (Acts 4:12) "So then faith cometh by hearing, and hearing by the word of God." (Romans 10:17)
You Must Believe
Hebrews 11:6
You must believe and have faith in God because “without faith it is impossible to please him: for he that cometh to God must believe that he is, and that he is a rewarder of them that diligently seek him.” (Hebrews 11:6) But neither belief alone nor faith alone is sufficient to save. (James 2:19; James 2:24; Matthew 7:21)
You Must Repent
Acts 3:19
You must repent of your sins. (Acts 3:19) But repentance alone is not enough. The so-called “Sinner’s Prayer” that you hear so much about today from denominational preachers does not appear anywhere in the Bible. Indeed, nowhere in the Bible was anyone ever told to pray the “Sinner’s Prayer” to be saved. By contrast, there are numerous examples showing that prayer alone does not save. Saul, for example, prayed following his meeting with Jesus on the road to Damascus (Acts 9:11), but Saul was still in his sins when Ananias met him three days later (Acts 22:16). Cornelius prayed to God always, and yet there was something else he needed to do to be saved (Acts 10:2, 6, 33, 48). If prayer alone did not save Saul or Cornelius, prayer alone will not save you. You must obey the gospel. (2 Thess. 1:8)
You Must Confess
Romans 10:9
You must confess that Jesus Christ is the Son of God. (Romans 10:9-10) Note that you do NOT need to make Jesus “Lord of your life.” Why? Because Jesus is already Lord of your life whether or not you have obeyed his gospel. Indeed, we obey him, not to make him Lord, but because he already is Lord. (Acts 2:36) Also, no one in the Bible was ever told to just “accept Jesus as your personal savior.” We must confess that Jesus is the Son of God, but, as with faith and repentance, confession alone does not save. (Matthew 7:21)
You Must Be Baptized
Acts 2:38
Having believed, repented, and confessed that Jesus is the Son of God, you must be baptized for the remission of your sins. (Acts 2:38) It is at this point (and not before) that your sins are forgiven. (Acts 22:16) It is impossible to proclaim the gospel of Jesus Christ without teaching the absolute necessity of baptism for salvation. (Acts 8:35-36; Romans 6:3-4; 1 Peter 3:21) Anyone who responds to the question in Acts 2:37 with an answer that contradicts Acts 2:38 is NOT proclaiming the gospel of Jesus Christ!
You Must Be Faithful Unto Death
Revelation 2:10
Once you are saved, God adds you to his church and writes your name in the Book of Life. (Acts 2:47; Philippians 4:3) To continue in God’s grace, you must continue to serve God faithfully until death. Unless they remain faithful, those who are in God’s grace will fall from grace, and those whose names are in the Book of Life will have their names blotted out of that book. (Revelation 2:10; Revelation 3:5; Galatians 5:4)