Musar zu Wajikra 5:21

ומעלה מעל בה' . Rashi, quoting Rabbi Akiva, comments that since transactions such as loans and the purchase of something substantial always involve the presence of witnesses and the recording of the transaction in a document, we must assume that denial in the quotation means that the guilty person denies the authenticity of the document and/or the reliability of the witnesses. This is not the case, however, when someone entrusts valuables to a friend. The essence of such a transaction is its very secrecy. The depositor can only rely only on the third party [G–d] to know the true facts. Any denial of such a deposit by its recipient is a direct sin against G–d. Thus far Rashi. This teaches us what enormous dimensions a simple monetary dispute may assume. It may be equivalent to denying the very existence of G–d, G–d forbid.