Maximal Ideal of Commutative and Unitary Ring is Prime Ideal

Theorem
Let $A$ be a commutative ring with unity.

Let $M$ be a maximal ideal of $A$.

Then $M$ is prime.

Proof
From Maximal Ideal iff Quotient Ring is Field:
 * the quotient ring $R / M$ is a field.

It follows from Field is Integral Domain that $R / M$ is an integral domain.

Finally it follows from Prime Ideal iff Quotient Ring is Integral Domain that $M$ is a prime ideal.