(as it is used here) means,
To say out loud;
to say you have
belief or faith in.
...The word is nigh
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
Romans 10:8, 9, and 10