"I don't know if this had already been mentioned here but would a Muslim be going to heaven if he was once a Christian believer?"
There is no true Christian who is now a Muslim. As the apostle John explained:
1 John 2:19 (NKJV)
19 They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us.
Anyone who claims to have once been a Christian but who is now Muslim (or an adherent to any other religion) was never a believer in the first place. Their denial of Christ merely reveals them for the "tare" that they are.
Again, not everyone who claims to be a Christian actually is one. As Jesus plainly stated,
Matthew 7:16-21 (NKJV)
16 You will know them by their fruits. Do men gather grapes from thornbushes or figs from thistles?
17 Even so, every good tree bears good fruit, but a bad tree bears bad fruit.
18 A good tree cannot bear bad fruit, nor can a bad tree bear good fruit.
19 Every tree that does not bear good fruit is cut down and thrown into the fire.
20 Therefore by their fruits you will know them.
21 "Not everyone who says to Me, 'Lord, Lord,' shall enter the kingdom of heaven, but he who does the will of My Father in heaven.
Real, genuine, saving faith is always evidenced in corresponding behaviour. This is what the apostle James points out very sharply in the second chapter of the New Testament book bearing his name. KKK members are not "bearing good fruit." No matter what they may claim about an allegiance to Jesus, their works show that their faith is not truly in the Good Shepherd who taught:
Matthew 5:43-45 (NKJV)
43 "You have heard that it was said, 'You shall love your neighbor and hate your enemy.'
44 But I say to you, love your enemies, bless those who curse you, do good to those who hate you, and pray for those who spitefully use you and persecute you,
45 that you may be sons of your Father in heaven...
No one who espouses the OSAS view supports the idea that salvation means one can live like the devil. I have heard OSAS preached and taught many times over the forty two years I've been a Christian and not once has it ever been said by those who so preached and taught that being saved was a license to sin. In fact, the very opposite is always preached. I don't understand why, then, those opposed to OSAS so regularly throw up this Strawman of the OSAS position.
I seriously doubt a genuine Christian believer will ever take the Mark Of the Beast. But even if they did, is God's grace so small that it cannot cover this sin? That's not what I read in Scripture:
Romans 5:20 (NKJV)
20 Moreover the law entered that the offense might abound. But where sin abounded, grace abounded much more,
Selah.