During an argument with his wife, he reviled Allah and reviled the religion, then after he was sure that he had gone beyond the pale of Islam and apostatised, he declared his repentance, uttered the twin declaration of faith (shahaadatayn) and expressed his great regret. Is this act of apostasy on his part regarded as breaking his fast during the month of Ramadan? If that is the case, does he have to offer expiation by fasting one day only, or must he fast for two consecutive months?

Praise
be to Allah

Being Muslim is one of the essential conditions for the
validity and acceptance of acts of worship. 

One of the indications of that is the verses in which Allah,
may He be exalted, says (interpretation of the meaning):


“Say: ‘Spend
(in Allah’s Cause) willingly or unwillingly, it will not be accepted from
you. Verily, you are ever a people who are Fasiqoon (rebellious, disobedient
to Allah).’


And nothing prevents their contributions from being accepted
from them except that they disbelieved in Allah and in His Messenger
(Muhammad (blessings and peace of Allah be upon him)); and that they came
not to As-Salat (the prayer) except in a lazy state; and that they offer not
contributions but unwillingly”


[at-Tawbah 9:53-54]. 

This verse clearly shows that the condition of an act of
worship being accepted is faith, without which it is not valid. 

Ibn Katheer (may Allah have mercy on him) said:

The words “Say:
‘Spend (in Allah’s Cause) willingly or unwillingly”
mean: No matter what you spend, willingly or unwillingly, “it
will not be accepted from you. Verily, you are ever a people who are
Fasiqoon (rebellious, disobedient to Allah).” 

Then Allah, may He be exalted, tells us of the reason for
that, which is that it will never be accepted from them, is “that
they disbelieved in Allah and in His Messenger (Muhammad (blessings and
peace of Allah be upon him)”
that is, they have disbelieved, and deeds are only valid when accompanied by
faith.

End quote from Tafseer Ibn Katheer (4/162). 

Whoever disbelieves and apostatises during his fast has
rendered his fast invalid. 

Ibn Qudaamah (may Allah have mercy on him) said: 

We do not know of any difference of opinion among the
scholars concerning the fact that the one who apostatises from Islam whilst
fasting invalidates his fast, and he must make up that day when he comes
back to Islam, whether he becomes Muslim again during the day or after the
day has ended, and whether his apostasy had to do with some belief that he
held, that constituted disbelief (kufr), or doubting a belief where doubting
that belief constitutes disbelief (kufr), or because he spoke words of
disbelief, either by way of mockery or otherwise. Allah, may He be exalted,
says (interpretation of the meaning):


“If you ask
them (about this), they declare: ‘We were only talking idly and joking.’
Say: ‘Was it at Allah, and His Ayat (proofs, evidences, verses, lessons,
signs, revelations, etc.) and His Messenger (blessings and peace of Allah be
upon him) that you were mocking?’


Make no excuse; you have disbelieved after you had believed.
If We pardon some of you, We will punish others amongst you because they
were Mujrimoon (disbelievers, polytheists, sinners, criminals, etc.)”


[at-Tawbah 9:65-66]. 

End quote from al-Mughni (4/369-370) 

With regard to expiation, we have previously explained in
fatwa no. 106476 that it is not required except in the case of one who
spoils the Ramadan fast by having intercourse only. As for all other things
that break the fast, nothing is required except making up that day only. 

Based on that, this man has to make up one day in place of
the day that he spoiled, and he does not have to offer expiation. 

And Allah knows best.