Parshat Vayikra

Vayikra 5:21-22
When a person sins and commits a trespass against the LORD by dealing deceitfully with fellow in the matter of a deposit or a pledge, or through robbery, or by defrauding his fellow, or by finding something lost and lying about it …
Ibn Ezra
[commits a trespass against the LORD] a breach of faith in God by doing something wrong to his fellow man
Rabbi Adin Steinsaltz
it is accurate to say that Leviticus deals with the various aspects of holiness. … Holiness is the context for all the subjects discussed throughout Leviticus.
… laws whose rationale appears, at first glance, to be related to law and order or to morality appear in Leviticus as deriving from the sphere of holiness. An example of this can be seen in the section on dishonesty [Leviticus 5:20-21] … Beyond what he did to his fellow man, he committed “a trespass against God.” This is a new factor, not a social factor but a kind of desecration. The sinner has desecrated something that was set aside as holy. 
It is important to stress that if the general common denominator in Leviticus is the theme of holiness, then the definition of holiness here is not exactly the definition we would expect. Holiness is not only what one does or does not do in the Temple, but something that applies even in places that have nothing at all to do with the ritual holiness of the Sanctuary or the Temple. It is a spiritual quality in its own right, beyond the kind of holiness described by the Maharal, for example, who speaks of holiness as the aspect of standing apart from everything or as a type of detachment (Tiferet Yisrael 11). Here, holiness diverges from the ritual sphere and takes on a different meaning: something special or unique.
From the book of Leviticus it follows that if an ordinary person steals, he, too impinges, somehow on holiness. To defraud someone is “to commit a trespass against God.” This may seem strange; what does stealing from one’s neighbor have to do with God? However, the Torah insists that such a person has committed sacrilege, and therefore must make amends before God.
What all this adds up to is that holiness is a type of general refinement, perfection, and exaltation …
You are holy, you are uplifted; therefore, you must not degrade yourselves and sink so low. The requirement of holiness in Leviticus is thus a type of musar. … “This kind of behavior is beneath you.”
the requirement of “You shall be holy, for I am holy” derives from the comparison of one’s heavenly image with one’s earthly image,  as though to say: This is your source, this is your root, you originate from this ideal image; in light of this — how can you possibly sin?
That is why we say each morning: “My God, the soul that You gave me is pure.” We start from above and continue below. 
Rambam, Guide for the Perplexed, Part 1 54:4
for the chief aim of man should be to make himself, as far as possible, similar to God: that is to say, to make his acts similar to the acts of God, or as our Sages expressed it in explaining the verse, “Ye shall…be holy”: “He is gracious, so be you also gracious: He is merciful, so be you also merciful.”