Ro 10:9 that if you confess with your mouth the Lord Jesus and believe in your heart that God has raised Him from the dead you will be saved.
The word "believe" here means to trust. When you trust in the fact that God has raised the Lord Jesus Christ from the dead you will be saved.
Why is that?
The resurrection of the Lord Jesus was the proof that God was satisfied with what Christ did on the cross for sinners. The resurrection was God in effect saying: "I have accepted the death of My Son for the sins of the world. Because of what He did on the cross the penalty was paid. To show you that I have accepted and received His work on your behalf, I will raise Him from the dead. If His death on the cross did not satisfy Me, then He would have remained in the grave. I would keep Him there. The ultimate penalty of sin is eternal death. But His death eternally paid for it and I am wholly and totally satisfied with what He did. Therefore I have raised Him from the dead."
If you believe and trust in the fact that God has raised Him from the dead and understand its implications, then that implies that you also trust that God was satisfied with what Christ did on Calvary's cross for your sins. That Calvary's cross satisfied God and therefore you put your whole trust in it alone for salvation.
Christ's resurrection was God's "Amen!" to Christ's "It is finished!"
And anyone who has genuinely trusted in Christ for salvation cannot stay silent about it. He just can't help it but confess Christ. What the heart is full of the mouth inevitably speaks. Just look at Paul:
2Co 4:13 And since we have the same spirit of faith, according to what is written, "I believed and therefore I spoke," we also believe and therefore speak
The mouth confession is the outward expression of the inward faith already in the heart.
