The Shahāda is built upon negation and affirmation: it begins with negation and concludes with affirmation.
Likewise, the Cross consists of two beams: one that stands firm by its own nature, and another that has no stability except through the stability of the first.

The Shahāda is four words, and the Cross likewise has four extremities.
The extremity fixed in the earth corresponds to the master of ta’wīl -interpretation of truth-, upon whom the souls of the seekers find rest.
The extremity that rises above, elevated in the air, corresponds to the master of spiritual support, upon whom the souls of the affirmed find rest.

As for the two horizontal extremities — to the right and to the left — they correspond to the tālī (the follower) and the nāṭiq (the speaker), one facing the other, one the master of composition and ordering.
And the upright extremity that extends from the sābiq is the one from which all letters are drawn and given extension.

No God Except Allah Muhammad is The Massenger of God

Except

إلا

الله

Allah

ٌمحمد

ُرسول

الله

Muhammad is The Massenger of God

God

إله

No

لا

The Shahāda is composed of seven sections; likewise, the Cross has four angles and three ends.
Each of its ends contains three points, completing the whole as twelve — just as the Shahāda itself consists of twelve letters.

And just as the Shahāda is perfected only by its conjunction with Muḥammad — Blessings be upon him and his Household— so too the Cross was ennobled only after the one belonging to that era was placed upon it.”*

“Al-Yanābīʿ — On the Correspondence of the Cross and the Shahāda — by Abū Ya‘qūb al-Sijistānī.”