This is a difficult question, but in the end we have to depend on Scripture.
Hell is an undeniable fact. For all the efforts to explain it away as not something a good God would do, Jesus had a lot of warnings about Hell. For example, in Matthew 8:12 Christ warned of a place of "weeping and Gnashing of teeth", and that this would be the 'subjects of the kingdom', that is, the people who arrogantly assumed they were safe from such a fate.
In Matthew 25:46 Christ warned that those who were hard of heart and cold to those in need would go to "eternal punishment".
Jesus strongly warned people not to fear even those who could kill their bodies, but they should "fear him who can destroy both soul and body in hell." (Matthew 10:28)
We would do well to consider the fate of Judas. It's easy to pretend none of us would betray Christ as Judas did, yet think about it: Judas followed Christ for years, heard what he taught, saw His miracles, and had access to the Lord such as no one today can even imagine.
Yet in the end, Judas chose according to his heart, and it destroyed him. Scripture even says Judas felt remorse when he realized what he had done, but even that did not lead to his redemption ... only his suicide and descent to hell.
We should not assume we will end up better just because we are not Judas.
Then again, there is hope in Scripture as well. Consider Ezekiel 18:19-23:
"Yet you ask, 'Why does the son not share the guilt of his father?' Since the son has done what is just and right and has been careful to keep all my decrees, he will surely live. The one who sins is the one who will die. The child will not share the guilt of the parent, nor will the parent share the guilt of the child. The righteousness of the righteous will be credited to them, and the wickedness of the wicked will be charged against them.
"But if a wicked person turns away from all the sins they have committed and keeps all my decrees and does what is just and right, that person will surely live; they will not die. None of the offenses they have committed will be remembered against them. Because of the righteous things they have done, they will live. Do I take any pleasure in the death of the wicked? declares the Sovereign Lord. Rather, am I not pleased when they turn from their ways and live?"
One important key to Scripture, is that it is consistent. God does not change His mind or alter His promises. The covenant God made with Noah was consistent with the one God made with Abraham, then David, and so on. It is rightly said that the Old Testament points ahead to Christ, and the New Testament back to Him. And the Redemption of Man through Christ is the power behind every promise of hope offered by God. And that hope is revealed through exercise of Faith.
It is wrong to claim that a man can go to Heaven if he does good works, because our sin cannot be washed away by our works. But faith in God, expressed in work, reveals God at work in us.
Consider that when Jesus spoke of the Good Samaritan, He spoke of a man who did not just say good words or offer a prayer, but took it upon himself to go out of his way to help a man he did not even know.
So coming back to the question of someone who professes faith but falls back into sin. The answer ultimately is that God knows what is in someone's heart, and we should work out where we are with the Lord, as the verse says, with 'fear and trembling'.
Believe in God but don't assume He is obligated to give you what you want just because your assumption is convenient. Many who play up their religion will find that God is not impressed with them, while others who seem to be of no significance to people, are treasured by God and will be welcomed into His House with great praise.
With all that said, keep in mind that I know no more than anyone else, and have a set of flaws and blunders to make anyone blush. Whether my words seem wise or not to you, I commend us all to depend more on Scripture and Prayer than an internet forum.