Consider a reaction:$$\ce{$aA + bB$ <=> $cC + dD$}$$

The value of reaction quotient at a certain time $t$, $${Q_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}}$$

where the concentrations $[A], [B], [C]$ and $[D]$ are at time $t$.

Let the reaction start initially at $t = 0$, with only reactants $i.e.$ $[A]$ and $[B]$ equal to say $1$ mol and $[C]$ and $[D]$ equal to $0$ mol. Hence, $$Q_c = 0$$

We know that the value of equilibrium constant $K_c$ must be such that, $$K_c > 0$$

Thus, $$Q_c < K_c$$

Which is the only condition for the advancement of reaction in forward direction. This condition does not consider the value of change in Gibbs energy $\Delta G$.

Now, considering the relation $$\Delta G = \Delta G^o + RT~\mathrm{ln}~Q_c$$

When $Q_c = 0+$ then $\mathrm{ln}~Q_c \to -\infty$, which means $\Delta G << 0$ and reaction is spontaneous in forward direction.

**Hence, can it be concluded that every reaction is spontaneous in forward direction if it starts with only reactants?**

If so, how can we define a non-spontaneous reaction?