Serving in the Temple requires a state of absolute perfection, demanding both spiritual and physical wholeness. The requirement for a priest to be completely without blemish is designed to maintain the honor of the holy service, reflecting a profound connection between the soul's readiness for its divine mission and the physical condition of the body. Interestingly, while many instructions are directed to Aaron and his sons together, this specific command is given exclusively to Aaron regarding his future descendants. The primary approach among commentators is that this distinction stems from a deep respect for Aaron. Because Aaron was holy and physically perfect, the text avoids any implication that he himself could be flawed, instead instructing him to warn future generations [רמב״ן, הטור הארוך, רבנו בחיי]. Furthermore, this phrasing contains a hidden divine promise: Aaron and his immediate sons were guaranteed to remain free of blemishes. The warning was meant for later generations, and even then, only a portion of them would be affected [אור החיים]. From a practical standpoint, the prohibition against serving with a blemish does apply to a High Priest, but this was concealed out of respect, teaching that one must not rely on miracles [רמב״ן, הטור הארוך].
Beyond physical wholeness, maturity is a prerequisite for service. Even if a priest is completely unblemished, he remains disqualified from the Temple service while he is a minor; he must reach adulthood before he can participate [תורה תמימה, חזקוני, אדרת אליהו]. Furthermore, the emphasis on Aaron's offspring clarifies that no level of distinguished lineage can override the disqualification of a physical blemish [חומת אנך].
A blemish is defined as any visible deficiency or alteration in the body's structure, even if it does not hinder the person's natural physical abilities [רלב״ג, ביאור יש״ר]. This includes any permanent flaw, whether the priest was born with it or acquired it later in life [תורה תמימה, מלבי״ם, אדרת אליהו]. The restriction against approaching the altar is absolute and extends beyond the actual offering of sacrifices. A blemished priest is forbidden from approaching the altar even for the purpose of training [העמק דבר]. He is also prohibited from assisting in secondary tasks that would otherwise be permissible for an ordinary person, such as pouring oil. Because he performs these actions in his capacity as a priest, having a blemished priest carry them out is considered a disgrace toward Heaven [ביאור יש״ר, רד״צ הופמן].
When referring to the offerings as the bread of God, the concept is not limited to actual baked goods like the showbread or meal offerings. The primary approach among commentators is that bread is used here in a much broader sense to mean food or a grand feast, encompassing all types of sacrifices offered on the altar [רש״י, מזרחי, אבן עזרא, משכיל לדוד].
Looking past the physical and legal dimensions, there is a profound spiritual meaning behind these disqualifications. A physical blemish can serve as a divine indicator that God knows the soul of this particular priest is not suited to act as His messenger for offering sacrifices [פרדס יוסף]. In some cases, a physical deformity is actually the outward manifestation of internal spiritual flaws, sins, or negative character traits. A person's inner spiritual decay can eventually result in a physical blemish that permanently disqualifies him from the priesthood [כלי יקר, שפתי כהן].