How Do I Talk to a Believer in an Non-Personal God?

Assuming that the person is willing to engage in rational discourse, I would encourage you to take him through St. Thomas Aquinas’s treatment on God’s knowledge in question 14 of the first part of the Summa Theologiae. The very first article is “Is there knowledge in God?” Inasmuch as there is intelligence in God, it follows that he is personal.

Of course, this presupposes other attributes about God that you may have to establish first, such as immateriality. Aquinas deals with this in article 1 of question 3 of the first part of the Summa.

Such a route of evangelism may seem daunting and abstract. But when we’re talking about the nature of God, we have to use the appropriate tools, one of which is philosophy.