Simply believing doesn't save a person. It is written that with the heart one believes into righteousness, and with the mouth acknowledgement is made into salvation
For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation. Romans 10:10
For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation. Romans 10:10
I've heard certain folks (especially those who attend the church of Christ) misinterpret Romans 10:9-10 in such a way that means we can believe unto righteousness today, but are still lost until we confess Christ, which may be next week and then we are finally saved next week, but that is not what Paul is talking about here. Also, someone who is mute (unable to speak) would remain lost according to that erroneous interpretation of Romans 10:9,10 for failing to "verbally confess with their mouth."
Confessing with our mouth that Jesus is Lord and believing in our heart that God raised Him from the dead are not two separate steps to salvation but are chronologically together. Romans 10:8 - But what does it say? "THE WORD IS NEAR YOU, in your mouth and in your heart" (TOGETHER) that is, the word of faith which we are preaching, (notice the reverse order from verse 9-10) - that if you confess with your mouth that Jesus is Lord, and believe in your heart that God raised Him from the dead, you will be saved; 10 for with the heart one believes unto righteousness, and with the mouth confession is made unto salvation. Confess/believe; believe/confess.
1 Corinthians 12:3 - Therefore I make known to you that no one speaking by the Spirit of God calls Jesus accursed, and no one can say that Jesus is Lord except BY the Holy Spirit. There is divine influence or direct operation of the Holy Spirit in the heart of a person when confessing Jesus as Lord. This confession is not just a simple acknowledgment that Jesus is the Lord (even the demons believe that), but is a deep personal conviction that Jesus is that person's Lord and Savior.
So simply believing in our head (and not in our heart) that God raised Him from the dead does not result in righteousness and simply giving "lip service" to the words "Jesus is Lord" not by the Holy Spirit is not unto salvation.