Say St. Thomas is right and the only knowledge we have of God is through his effects. We might wonder if this is enough to give rise to a religious response. If we only knew a person though his effects (like the art he produced or the money he donated) we might respect him, be amazed at him, and/or feel deep feelings of gratitude towards him, but it is not clear what we would think about making a personal commitment to him – it is not even clear what this would mean or whether it is possible. Again, it is not clear what it would mean to be in the service of a person who is only known to you through his effects.
(I waver over whether Romans 1 is a response to this.)