Let us remember that the speaker of this poem appears to be addressing a cynic of love, who is arguing that the speaker's love for his beloved is harmful or at least damaging to himself. Thus it is that the title of the poem refers to the way that the speaker and his beloved can become "canonised" or be made saints through the way that their love is expressed in poetry. Consider how the penultimate stanza of this excellent poem explores the paradox between the actual nature of their relationship and the way that it can be immortalised in poetry:

We can die by it, if not live by love, 
And if unfit for tomb or hearse
Our legend be, it will be fit for verse ; 
And if no piece of chronicle we prove, 
We'll build in sonnets pretty rooms ; 
As well a well-wrought urn becomes
The greatest ashes, as half-acre tombs, 
And by these hymns, all shall approve 
Us canonized for love ;

Even though the "legend" of their lovemight be "unfit for tomb or hearse," it can be "fit for verse." Poetry seems to be an apt way to give them the "canonisation" for the intensity of the love they bear for one another, even though in life they may receive no recognition for it at all. The paradox of the title refers to the difference between the way their love is thought of by others and the way it can be immortalised in poetry. The fact that we continue to talk about this poem so much nowadays gives testament to the way in which Donne achieved the "canonisation" of the love between the speaker and his beloved.