Stability at loop gain <1 and 180 degree phase shift
When the gain is >1 with 180 phase shift and another 180 by negative feedback , you have a positive feedback with growing amplitude since  loop gain >1 and non inverting and thus an oscillator. ( i.e. unstable or "astable" )

Where the loop gain >1 at 180 phase shift is the resonant frequency of the oscillator.

The output will be saturated which reduces the gain to 0 when saturated but by this time the stored energy ( sine voltage  feedback) from the reactance that created the 180 phase shift , has enough amplitude.  The DC still is negative feedback so it biases the into near 50% threshold for a square wave out.