What does it mean: To believe? Whom to believe? And what? Paul sums it up, concisely, in Romans 10:9.
If ever you should be avowing with your mouth the declaration
that Jesus is Lord,
(Lord of whom? "Lord of all!"; Rom.10:12)
and should be believing in your heart
(That is, out of love and not forced or out of dread)
(Jesus did not rouse Himself)
(a true happening – a historical fact)
from among the dead
(Jesus was dead, really dead)
you shall be saved.
(God is the "Savior of all mankind" and will bring everyone to that realization; Phil.2:10,11; 1Tim. 2:4; 4:10).
Translation: Peter Feddema