  The main aim of this paper is to demonstrate the method called "the
Bosonization of Nonlocal Currents" (BNC), used for calculations of bound states
in a quark model, within the simplest relativistic quantum field model of two
scalar fields with the Yukawa type interaction. A second aim is to clarify the
relation between BNC and two widely used methods, employed in recent particle
physics to calculate bound states of interacting particles, based on the
nonrelativistic Schrodinger equation (the S-method), and the relativistic
Bethe-Salpeter equation (the BS-method), and to determine the conditions on
parameters of a quantum field model dictating a definite method to be applied.
  It is shown that all these methods can be applied only in the weak coupling
regime when the effective dimensionless coupling constant should be less than
1. The basic parameter separating the relativistic and nonrelativistic pictures
is the ratio of the masses of the exchange ("meson") and constituent
("nucleon") particles. If this ratio and the coupling constant are small then
the potential picture takes place, i.e., the bound state is described by the
nonrelativistic Schrodinger equation. Otherwise, the Bethe-Salpeter equation or
the BNC should be employed. One notes that the BNC method has a slightly wider
region of applicability in this case.
