Following in the steps of George, Thomas continued to send delegations to the Franks, taking advantage of the treaty of friendship that had been formed by Charlemagne with the Abbasid Caliph Harun al-Rashid of Baghdad.
While Thomas was patriarch, Jerusalem was infested by locusts that caused a famine and the temporary departure of the Muslims.
Promising to pay a penalty of a thousand gold coins, Thomas was released and, also, was able to keep the cupola intact.
He supported the theology of the Christian faith as expressed during the Fourth Ecumenical Council in a letter to the Armenians.
He also opposed Byzantine Iconoclasm and the teaching that the Holy Spirit also proceeds from the Son, the filioque, that was spreading in the Western Church.