"whereas Watts and Wesley and Cowper's Olney collaborator John Newton are uttering their hymns as it were from the pulpit, Cowper is one of those who sit at their feet, reporting faithfully how it seems to him, there, in the pew. Perhaps the clearest example is 'Sometimes a light surprises' (no. 162). One may have read this poem, or more probably sung it, many and many a time before realizing that the crucial word in it is the first. 'Sometimes'—only sometimes, not always, not even very often! The 'holy contemplation' that is thereafter evoked, the sweet security, the unforced adoration—all this is distinctly not what any one, it seems, should expect to experience at all often, in church or out of it. It is not presented as the normal condition of the Believer. Above all therefore it is not the pay-off, the guaranteed reward for going to church and trying to behave well. On the contrary one earns such fitful and infrequent benefits (though 'earn' is the wrong word anyway, for a Calvinist such as Cowper) only by first suffering through afflictions and desolations—the 'season of clear shining' comes only 'after rain', only 'when comforts are declining'; and there is no guarantee that it will come, even then. Similarly, one sings anyway; the consoling words that one sings strike dully and inertly Sunday after Sunday (one is even, so some might say, 'insincere' in singing them); it is only sometimes, on one or two Sundays out of many, that 'a light surprises' and the words take on heartfelt meaning, 'while he sings'. . . . An evangelist, or even the earnest incumbent in the pulpit, would surely prefer not to have the insecurity of the whole arrangement announced quite so bleakly. . . . [But] once one has taken the force of 'sometimes', the poem affects a fusion of states of feeling that one would have sworn were incompatible: of grateful security ('But he will bear us through') with wistfulness, even weariness, almost resentment. The opening word of the poem casts a beam of pathos and qualification across everything that comes after."
Donald Davie, "Introduction," The new Oxford book of Christian verse, ed. Donald Davie (Oxford: Oxford University Press, 1981), xxv-xxvi.