One may be reprobate, but one is not to assume this in deciding how to live. All the more so, since “the decrees of predestination are unknown to us,” as the 1553 edition of Article 17 says. Interesting use of the plural “decrees” there. 

In my former article on The Darker Side of Predestination, I spoke about how Article 17 of The Thirty-nine Articles is not entirely limited to talking about the positive aspects of that doctrine. It does indeed mention “the sentence of predestination”, the flip side of the coin, as do other Anglican formularies. Article 17 teaches that reprobation cannot be used as an excuse for immorality, not that there is no such thing as reprobation.

This reading is confirmed I think by a passage in Archbishop Thomas Cranmer’s proposed canon law reform, the Reformatio Legum Ecclesiasticarum. There, he writes,

On the fringe of the church there are many who live in a wild and dissolute way, who when they get interested in the subject, being dissipated by excess and completely cut off from the Spirit of Christ, always toss predestination and rejection, or (as they usually call it), reprobation, into their speech, arguing that since God by his eternal counsel has already determined something, both concerning salvation and destruction, they have some excuse for their wrongdoings and crimes and all manner of evil. And when pastors upbraid their dissipated and disgraceful life, they blame God’s will for their crimes and by that defence consider that the reprimands of admonitions are wasted… Wherefore everyone must be warned by us that in undertaking actions they should not rely on the decrees of predestination, but adapt their entire way of life to the laws of God, and contemplate that both promises to the good as well as threats to the bad are generally set forth to him in the Holy Scriptures.

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