You call them by their father’s name, that is justice with Allah; if you don’t know their father, they are your brothers in religion and your friends; there is no blame upon you for calling them unintentionally, but you will be held for calling them with intention in your hearts, and Allah is the Forgiving, the Merciful.
Before revealing this verse, the adopted sons were called suffixing their foster father’s name. By revealing this verse it is banned to call so. But what they had called before is forgiven to them. If you don’t know who their father is, you consider them as your brothers and friends in religion. After revealing this verse if anyone calls them so as before intentionally, he will be punished. See explanation 2: 286; 5: 89; 16: 89.