The pronoun in “if they turn away” refers to the disbelievers, and in the word “then know” it is for the believers. Meaning: So if those cruel tyrants turn away from faith, then trust, O believers, that “Ali” is your Mawla (protector), meaning your lord and your helper.
The word Mawla has become a proper noun for the Commander of the Faithful—sublime is his remembrance—by which he is indicated, due to the saying of the Master Messenger—His might was mighty: “Whomever I am his Mawla, Ali is his Mawla.” And the saying of the second, following the pledge of allegiance at Ghadir Khumm: “Bravo, bravo to you, O son of Abi Talib! You have become the Mawla of every believing man and believing woman.” 1 And the believers' addressing of him in their gatherings with him: “O our Mawla.”