I prayed for a number of years in a mosque behind an imam who used to revile Allah, may He be exalted, and I used to advise the imam not to commit an act of kufr and revile Allah, but he did not listen to the advice. I prayed behind that imam for nearly 5 years, after which I finally stopped praying behind the imam.

What must I do so that Allah will accept my repentance? Do I have to make up those prayers or not? What is the expiation for that?

Please note: I was aware of the situation of the imam of the mosque, and that he was a follower of innovation that constitutes kufr. During that time I had the keys to the mosque and I used to open the door of the mosque for the sake of Allah, may He be exalted, so that the worshippers could pray. I did not check the ‘aqeedah of the imam of the mosque; rather it was his habit to revile Allah, may He be exalted, a great deal.

Praise
be to Allah

Whoever reviles Allah, may He be exalted, or His Book or any
of His angels or Messengers is a disbeliever who is beyond the pale of
Islam; it is not permissible to pray behind him because the ummah is
unanimously agreed that it is not permissible to pray behind a disbeliever,
whether he was originally a disbeliever or is an apostate. The one who prays
behind him when he is aware of his situation has to repeat the prayer. 

Imam ash-Shaafa‘i (may Allah have mercy on him) said: 

The disbeliever cannot be an imam under any circumstances,
whereas the believer may be an imam in all circumstances, except that he
should not pray unless he is in a state of purity. 

Similarly, if a man is a Muslim and he apostatises, then he
leads the prayer whilst he is an apostate, the prayer of those who prayed
behind him is not acceptable unless he openly expresses his repentance
verbally before leading them in prayer. If he openly expresses his
repentance verbally before leading them in prayer, then it is acceptable for
them to pray with him.

End quote from al-Umm (1/195) 

Ash-Shiraazi said in al-Muhadhdhab (1/183): 

It is not valid for a disbeliever to lead the prayer, because
he is not one of those who are obliged to pray; if he goes forward and leads
people in prayer, he is not regarded as having become Muslim, because that
is one of the minor issues of faith, so he does not become Muslim by doing
that. The same ruling applies if he were to fast Ramadan or give zakaah on
his wealth. As for the one who prays behind him, if he is aware of his
situation, then his prayer is not valid, because he connected his prayer to
a prayer that is invalid…. End quote. 

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

It is not valid to pray behind a disbeliever under any
circumstances, whether one becomes aware of his being a disbeliever after
finishing the prayer or before that. The one who prayed behind him has to
repeat his prayer. This is the view of ash-Shaafa‘i and ashaab ar-ra’y. Abu
Thawr and al-Muzani said: The one who prayed behind him and was not aware of
the situation does not have to repeat his prayer, because he followed the
prayer of one whose situation he was unaware of, so his case is like that of
one who followed in prayer a person who is in a state of minor impurity.

End quote from al-Mughni (2/16) 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 

It is not valid to pray behind a disbeliever in any
situation, whether his disbelief is in terms of beliefs, words, actions or
things that he fails to do. 

In terms of belief e.g., if he believes that there is another
god besides Allah. 

In terms of words e.g., if he ridicules Allah or His
Messenger or His religion. Whoever ridicules Allah or His Messenger or His
religion is a disbeliever, even if he prays. … We know that no Muslim can
pray behind a disbeliever, but if we assume that someone prayed behind a man
and was not aware that he was a disbeliever until after the prayer was over,
does he have to repeat the prayer or not?

The answer is that some of the scholars said that he does not
have to repeat the prayer, because he is excused.

And some scholars said that he does have to repeat the
prayer, because one of the conditions of leading the prayer being valid is
that the imam should be a Muslim.

If someone were to say: can we differentiate and say that if
the signs of disbelief are obvious in his case, then the prayer is not
valid, and there is no excuse for ignorance because there is circumstantial
evidence, otherwise the prayer is not invalid?

Our answer is: that may be the case. The more correct
scholarly view concerning this issue is that if he is unaware (of the imam
being an apostate), then his prayer is valid.

End quote from ash-Sharh al-Mumti‘ (4/220).

Based on that, the prayers that you offered behind that
evildoer imam are not valid on his part, primarily because of his being an
apostate, or on your part, because your prayer was connected to the prayer
of an apostate. So you have to repeat all of these prayers, and strive hard
to work out how many they are, until you think it most likely that you have
done what you have to do. This is the view of the majority of scholars.

Ibn Qudaamah said in al-Mughni (1/439):

If the number of missed prayers is great, he should try hard
to make them up, unless that will cause him physical or financial hardship.
As for physical hardship, that is when he may get too tired or may fear
sickness. As for financial hardship, that is when it could lead to him
becoming unable to dispose of his wealth (and run his business), to the
point that it keeps him away from earning a living, or he may be harmed by
that. Ahmad stated something to this effect. If he does not know how many
prayers he owes, he should keep making them up until he is certain that he
has made up all that he missed. Ahmad said, according to the report of
Saalih, concerning a man who had missed prayers: he should keep making them
up until he is sure that he has made up all that he missed. End quote.

End quote.

This case is not like that of one who is committing an
innovation that could put him beyond the pale of Islam, because he may be
excused by some as he is basing his innovation on some sort of ijtihaad or
misinterpretation or the like, and even if he is to be regarded as a
disbeliever, his disbelief is not like that of one who has the audacity to
revile Allah, may He be glorified and exalted. So how about if he combines
these two evil deeds? How could such a person pray to the Lord of the
Worlds, let alone become an imam?!

Please see also the answers to questions no.
20885 and 129407

And Allah knows best.