If someone asks them:

Why do you not say, “A disbeliever, if Allah wills,” meaning complete disbelief, just as you say, “We are believers, if Allah wills,” meaning complete faith?

They reply:

Because a disbeliever denies the truth, whereas the foundation of a believer’s faith is acknowledgment. Denial has no initial and final stages requiring one to await its full realization. Faith, however, is founded on affirmation and acknowledgment: one must await the actual fulfillment of what has been acknowledged and the realization of what has been affirmed.

This is like two men who owe something to a third man. He asks one of them for what he is owed, and the man says, “I owe you nothing.” He thus denies and repudiates the claim. Once he has denied and repudiated it, no further stage remains by which he must substantiate what he has said. The creditor then asks the other man for what he is owed, and he replies, “Yes, I owe you such and such.” Yet his acknowledgment alone does not give the creditor his due unless he pays it in full. The creditor therefore waits for him to substantiate his words through payment and confirm his acknowledgment by fulfilling his obligation. If he acknowledges the obligation but then fails to give the creditor his due, he is effectively like the one who denied it, since both have equally failed to pay. Substantiating his words therefore means giving the creditor his due. If he pays part of it, he substantiates part of what he has said and fulfills part of what he has acknowledged. With every portion he pays, he further substantiates his acknowledgment. Likewise, the believer must continue fulfilling what he has acknowledged until he dies.

That is why we say, “A believer, if Allah wills,” but do not say, “A disbeliever, if Allah wills.”

Muhammad ibn Nasr said:

Another group among the people of hadith held a similar position, except that they called such a person a Muslim because he had left the religions of disbelief and acknowledged Allah and what He has said, but they did not call him a believer. They maintained that, although they called him a Muslim, he was a disbeliever—not a disbeliever in Allah, but a disbeliever in respect of his deeds.

They said:

This is disbelief that does not take a person outside the religion.

They also said:

It is inconceivable that the Prophet, may Allah bless him and grant him peace, should say:

“The one who commits unlawful sexual intercourse is not a believer while committing it.”