"The first verse of 'Shema,' 'Hear (shema) O Israel, the Lord our God the Lord is One (echad),' contains the main principles of Jewish faith.
To utter it with concentration and intention is to 'accept the yoke of Heaven.'
When this verse is written in Torah scrolls, two of the letters appear large: the 'ayin' of 'shema' and the 'daled' of 'echad.'
Allegorical interpretations of the Torah point out that when these two letters are put together in the order of their appearance, they make up the word "ed" (witness).
This refers to the Israelites’ testimony to God’s kingship, as it says in Isaiah 43:10: 'You are my witnesses, says the Lord.'
And when read in the opposite direction, they make up the word 'da' (know), as it says in Chronicles I 28:9, 'Know the God of your father, and serve Him with a perfect heart.'"
From "The Portion of Shema" (Deuteronomy 6:4-8) by Rabbi Adin Steinsaltz