Kierkegaard's Works of Love - Part II - Love Believes All Things and Yet is Never Deceived

So how is that love can believe all things and also that the lover is never deceived? This position is contrasted to mistrust, which believes nothing. Mistrust attempts to constitute itself on the basis of knowledge and so belief is irrelevant giving rise to disbelief. The point that SK wants to make is that disbelief is an active position not a result of knowledge. Both mistrust and love acknowledge that deception stretches out as far as the truth and so while it is always at risk of being deceived, it is still always possible to believe.

SK responds to this acknowledgment,

And so it is; so it shall be. Precisely because existence will test you, test your love or whether there is love in you, for this very reason with the help of the understanding it presents you with truth and deception as two equal possibilities in contrast to each other, so that there must be a revelation of what is in you since you judge, that is, since in judging you choose. . . . [Judgment] takes place every moment, because existence judges you every moment you live, inasmuch as to live is to judge oneself, to become open. . . . When deception and truth are presented as two equal possibilities in contrast to each other, the decision is whether there is love or mistrust in you.

Wherever truth may be, deception is also possible even in the purest of motives, feelings, or rationales. And conversely, "what appears to be the vilest behaviour could be pure love."
Love has no more access to knowledge than mistrust. It is only the decision that separates them. And knowledge offers only possibilities (even contrasting) not realities. Mistrust believes nothing and so logically it seems that it cannot be deceived. But mistrust has no greater claim to knowledge and so belief must also speak up and say to mistrust that it is being deceived out of the blessedness of love. But surely love, in some circumstances, can also be deceived. This is wrong. Love is the highest. The highest cannot be deceived. So to remain in love, which believes all things, is to never be deceived. And so the only possible deception is self-deception, to be deceived out of love, to become mistrustful.
Where love may be 'deceived' is the lower order of love which is market driven. Love as supply and demand. In this way one might swindled out of love. They have given love but received no love in exchange, they were cheated. This depends on SK's original sketch of love as love of neighbour and not erotic love or friendship. In the lower order of love it makes perfect sense to navigate both mistrust and 'love'. The lower order of love is a temptation to be guarded against.
The true lover is reconciled internally to love and understands the misunderstanding that may occur around him (being mocked as a fool). In all cases so long as the lover preserves herself in love she will not be deceived (for love is the highest good). Only the deceiver remains deceived as he cheats himself out of the two greatest goods which are to love in truth and to be loved in truth. The true lover has surrendered, offered, his love and so he cannot by definition be deceived out of it.
The erotic lover is ashamed to go loving the deceiver but the true lover "regards it as a victory if he might only succeed in continuing to love the deceiver." SK offers this by way of comparing the true lover and the deceiver,

Do you know, my reader, any stronger expression for superiority than this, that the superior one also has the appearance of of being the weaker? The stronger who looks like the stronger sets a standard for his superiority; but he who, although superior, appears as the weaker negates standards and comparisons - that is, he is infinitely superior.

This appears to me to be the existential position par excellence. That one is so reconciled to himself (in the God-relation for SK) that deception becomes literally impossible for one is already existing in the highest order, the infinite order. And so any possible deception is only ever self-deception.
As a side note. The only unease I had with this chapter was around the idea of a love that suffers abuse, that is, in an abusive relationship. However, what must be understood is that SK is always speaking in terms of the love of neighbour. Erotic love can indeed be corrupted. In this way a person may break from the erotic relationship while continuing to 'believe all things' which overcomes the drive for retributive justice and holds open the possibility of redemption, eternally.