John Calvin Commentary Exodus 20:26

"Neither shalt thou go up by steps unto mine altar, that thy nakedness be not uncovered thereon." — Exodus 20:26 (ASV)

Neither shall you go up. When God had prescribed modesty to the priests in their whole life and in their private actions, it is no wonder that He also required special care for decency and propriety in the performance of their sacred duties. He had indeed already desired that the priests should wear drawers or breeches when they went into the sanctuary; yet not content with this symbol of purity, He forbids them to ascend the altar by steps, lest perhaps the drawers themselves might be seen; since the dignity and sanctity of sacred things would thus be impaired. By all means, therefore, He would induce the Israelites to conduct themselves most purely and most chastely in their religious practices.