Scientists do not practice a faith. No scientist "believes in
gravity." However, every scientist knows that any theory of matter must
be able to account for the same things that theories of gravity can
account for.

   When you refer to gravity in your passage, you refer to the theory of
gravity that Newton outlined. It stated that if a force of gravity
exists betwen two objects that have mass, then regardless of how far
separated they are, they will instantaneously feel some attractive force
that is subject to the inverse-square law. This could be tested on earth
and it could be tested by looking up at the skies. You could assume its
truth and then with that assumption find its implications, and then you
could test those implications. It could easily be scrutinized, and it
was. The best collection of the rules of Newtonian mechanics came from
Newton's Principia, which Newton made so mathematically rigorous
that many physicists had a hard time reading it. Newton wanted to make
sure that there was no one who could find mathematical (and logical)
objection to his conclusions. His theories became well-known because
their predictions were so accurate. They had implications no one even
imagined. People found themselves climbing out on mountains trying to
make the most precise measurements just to find that their experiments
were consistent with Newton's theory. Newton's theories held up to
scrutiny. . . But no one claimed that his theories were some snapshot
of "reality." Any theory of "reality" would have to be just as good
at explaining the physical world as Newton's theories, but no one was
seriously looking for such a theory.

   But then near the time Einstein entered the scene, people found problems
with Newton's ideas. Others had measured the apparent speed of light,
and Maxwell had shown light must be an electromagnetic wave with a finite
maximum velocity. Together, this meant that if the moon was suddenly to
disappear, the oceans on earth would notice it before our eyes did. By
this time, the theory of special relativity was avialable, and this
Newtonian implication violated special relativity. Observations
supported both special relativity and Newton's theory of a gravitational
force, but Newton's theory of a gravitational force contradicted
special relativity, which FOLLOWED FROM the assumption that the
speed of light was the maximum speed of anything in the universe. This
meant that either something was wrong with Newton's ideas or something
was wrong with that special relativity assumption.

   So at this point in science, scientists believed that one of their
theories was wrong, and thus a theory needed to be modified or
removed. This wasn't like changing to a new religion. This was a simple
matter. It might mean a lot of work will follow to correct the mistake,
but it had to be done. There was no question of that. Science could not
continue without fixing this mistake.

   And so Einstein had to come up with a theory ("general relativity") that
would hold up to both. It would explain gravitational experiments
and would not contradict special relativity. His
theory rejected the idea of a gravitational force. This
resulting theory would complete (not replace) Newtonian mechanics
by fixing this little hole. With this modification, the combination of
Newtonian mechanics and relativistic mechanics (by the way, Einstein
never wanted to use the word "relativity," he wanted to use the word
"invariance," referring to the invariance of space-time) could explain
all celestial and terrestrial motion without any contradiction.

   Relativistic mechanics states that gravity is not a force that causes
matter to be attacted to each other. It states that if objects
exist in a four-dimensional configuration space, then all curved
trajectories produce the same effect. Thus, if this configuration
space already possesses some curvature, the resulting trajectory will
be equivalent to the impact of an acceleration. In other words, moving
at a constant speed around a circle is an acceleration, as it requires
moving in a curved trajectory in the x,y,z subspace. However, moving in
a straight line (in the x direction, for example) while speeding up is
also an acceleration, because it requires traveling in a curved line on
the x-t plane. (imagine t on the horizontal axis and x on the vertical;
a parabola is etched out) Thus, we can re-define an accelerating object
as something that follows a curved trajectory in this four-dimensional
configuration space. Gravity thus could be explained as a natural
curvature of space-time caused by the presence of matter (this idea
was inspired by the ideas of Ernest Mach, the philosopher physicist who
came before Einstein, for whom the speed of sound is named).

   Notice that in both theories some necessary conclusions
follow from a statement. These conclusions can be tested. If these
conclusions do not hold, then the statement must not be true. That's
it. There's no act of belief there. No one "believes in" science. Science
follows naturally from reasoning.