"I believe in God because I feel his presence."
Always keep in mind that you will almost never win an argument WHILE it's happening. You may influence onlookers. And your partner (a term which some may find preferable to "opponent") may give honest and earnest consideration to your points upon review. That being said, how might we respond?
Seek clarity. Theists tend to dodge by changing the subject or definition when their suppositions are deflating. Here too, any direct assault will be like squeezing a wet watermelon seed. In seeking clarity regarding the "I feel" pseudo-evidence try to establish why the claimant believes that statement is meaningful.
A possible question to ask: "Should I (or anyone else) believe because of what you feel?" If the answer is "yes" then we might ask why this single perception of presence countermands the presence perceived by adherents of non-abrahamic faiths both today and historically. If the answer is "no" then we might ask how they can recognize that their perception of presence is insufficient second-hand evidence but as the object of the unreliable subjective experience they feel justified in accepting it as irrefutably divine.
The senses can be fooled and the psyche may be led astray.
The same experience of "divine presence" is available through every faith system, electromagnetic bombardment of the brain, neural chemical imbalances, physical brain damage, intoxicants, mental illness and the good ole natural high (usually brought on by awe, wonder or enthusiasm). That such a feeling is so commonly a result of religious fervor should not be considered "inspirational" as much as it is should be considered "embarrassing."
In every other realm of the human experience, loss of bodily function or mental faculty is either tragic or comical. But we've bound this psychological handicap to culturally accepted intangible forces that defy all objective observation. Somehow this disguises our pitiful and primitive misunderstanding of the world (and of ourselves) with a euphoric facade of exalted cosmic self-importance.
What is "felt" does not indicate substance. What is "felt" can only influence valuation. If you don't believe me just ask every girl I thought I was in love with as a teenager.