When a person rationalizes his sins, the personal dishonesty might causes additional distortion beyond the original sins, with even more toxic effects.  Our Gemara on Amud Beis quotes a verse (Devarim 19:15) that is used to describe the powers and limitations of a singe witness versus a pair:

לֹֽא־יָקוּם֩ עֵ֨ד אֶחָ֜ד בְּאִ֗ישׁ לְכׇל־עָוֺן֙ וּלְכׇל־חַטָּ֔את בְּכׇל־חֵ֖טְא אֲשֶׁ֣ר יֶֽחֱטָ֑א עַל־פִּ֣י ׀ שְׁנֵ֣י עֵדִ֗ים א֛וֹ עַל־פִּ֥י שְׁלֹשָֽׁה־עֵדִ֖ים יָק֥וּם דָּבָֽר׃

A single witness may not validate against an [accused] party any guilt or blame for any offense that may be committed; a case can be valid only on the testimony of two witnesses or more. 

Agra Dekallah darshens this verse to underscore the side effects of sin.  He states that since we know that reading Shema every day is testimony of our faith and belief in God, to properly fulfill this mitzvah, one must be a valid and kosher witness.  This idea is represented by the two larger letters in the Torah in the verse of the Shema, Ayin and Dalet, which form the word “Eyd” - witness.  If a person commits sins that disqualify him as a witness, his Shema is also in jeapordy.  This is hinted at in the words, “A single witness may not testify for any offense that may be committed”, that is, the sinner cannot stand up and declare that God is one in Shema.  Only through “two or three witnesses can the matter stand”, that is, the person must be able to testify through the Shema twice or three times daily (morning and evening prayers, and the Shema before bed.)

While this is an interesting derash, what does it really mean?  Why does sin disqualify the Shema?  I believe it is because if one is so wrapped up in self-deception that he is committing fraud, then his declarations of faith are suspect and cannot be valid.  How can a person profess devotion when he is fundamentally dishonest?