To say out loud; to say you have belief or faith in.

...The word is nigh thee, even in thy mouth, and in thy heart; that is, the word of faith, which we preach. That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised Him from the dead, thou shalt be saved. For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation.
Romans 10:8, 9, and 10