The speaker goes looking for love and comes up short. He searches his pocket and finds only "a little god" and his hat, which he says he is "glad to eat." The title, "Substitutionary," points to the idea of one thing standing in for another, and the poem plays with that: a small god takes the place of love, and a hat stands in for a meal.
The tone is dry, wry, and a little rueful. "Eat my hat" is an English phrase for admitting you were wrong, so the speaker may be conceding that he expected more than he got. Yet he says he is glad, which turns the loss into something close to a joke. Calling the god "little" shrinks big ideas like faith and love down to pocket size, and the word "abstract" suggests love is hard to find when it stays an idea rather than something you can hold. The mood is humble, funny, and quietly bittersweet.