Is it permissible to describe someone who has believed in Tawhid but has not yet submitted as "my brother"?

A disbeliever does not enter Islam except by uttering the two testimonies of faith (Shahadatayn) along with believing in them. If he does not utter them while being able to do so, he remains a disbeliever. It is not permissible to address a disbeliever as "my brother," because there is no brotherhood between a Muslim and a disbeliever. True brotherhood is the brotherhood of faith, as Allah Almighty says: "The believers are but brothers." One should strive to invite the disbeliever [to Islam] with gentleness, kindness, and good treatment, so that Allah may guide him.