If we don't want to make a mistake in asking if an article that describes the perfection of Christ is even necessary, we must recall the narrative nature of the Thirty-Nine Articles. Article 15 has been implied previously in Article 2. Article 9 explained our depth of our sinful nature and how that original sin continues to infect the regenerate believer. Article 11 sets out our justification in Christ’s merit alone. Articles 12-14 explained the nature of our good works. Sin, in its very nature, is blinding, so we don't detect its presence. Cranmer’s pastoral and theological genius we see here is to reorient the believer in Christ alone in whom all our hope is found and to remind us that the presence of sin that remains must be put to death every day.
XV—Of Christ Alone Without SinChrist in the truth of our nature was made like unto us in all things, sin only except, from which he was clearly void, both in his flesh, and in his spirit. He came to be the Lamb without spot, who, by sacrifice of himself once made, should take away the sins of the world, and sin, as Saint John saith, was not in him. But all we the rest, although baptized, and born again in Christ, yet offend in many things; and if we say we have no sin, we deceive ourselves, and the truth is not in us.