Easiest way to prove integral of $e^{ikx}$ is $\delta(k)$

Question

What is the easiest way to to derive the following equation:

$$\int_{-\infty}^{\infty}e^{ikx}dx = 2\pi\delta(k)$$

I understand the equation can be derived by assuming the Fourier integral theorem, but can anyone derive this without making reference to the Fourier integral theorem?