Who determines if a person goes to heaven or hell?
Who else, but God?
Does God looks at the heart of people in making His call?
God looks at the heart in relation to what?
The "heart" simply refers to a person's attitude, but one's attitude does not exist in a void/vacuum; since the attitude is always relative to something real and tangible.
As regarding the Ontology of Salvation, one's attitude is always relative to (1) God and (2) His fellow human. This is clearly mentioned in Matthew 22:37-40, Mark 12:30-31, and Luke 10:27, which is the basis of the new covenant.
Now, Jesus said that (1) is the first commandment, which takes precedence over (2) - Matthew 22:38 and Mark 12:30.
That is, the first commandment (1) is to love God with all your heart and with all your soul and with all your mind and with all your strength - Matthew 22:37, Mark 12:30, and Luke 10:27 - Which takes priority over the second commandment (2) to love your fellow humans.
But how does one "love" an intangible, invisible God whom you cannot see, nor touch, nor listen to his voice?
By demonstrating one's love towards Jesus Christ Son of God (or God made flesh) who is a tangible and visible being = Romans 10:9.
But to "love Christ" means that one should (a)
Confess allegiance with the LORD Jesus
and also (b)
Believe in his Resurrection - Which taken together is the new covenant of Christ (also, called the Covenant of Salvation) = Romans 10:9 - As it is clearly stated that both (a) and (b) are necessary conditions for any Christian to be SAVED.
Therefore, to "
Love Jesus" means to "
Honour the Son of God" - Since the word "love" is nuanced, and its various meanings are nuanced by using the same word in the English language; although in the Greek language there are different types of "love", which are ascribed different words.
So, to "Honour Jesus Christ" is the whole point about (a) and (b) in Romans 10:9.
So, by using Romans 10:9, one can make the following inferences:-
(I) The actual numbers who survive the Great Judgement to enter the Kingdom of heaven is exactly the same as the numbers who Believe in the Resurrection of Christ; inasmuch as X numbers will "Honour the Son of God", then the same numbers would be saved to enter the Kingdom of heaven.
(II) So, assuming that all people were to Believe in the Resurrection of Christ -
Either, with or without knowledge of the Covenant - then, all people would be saved, and not a single soul should be lost.
(III) But if only one person were to Believe in the Resurrection of Christ, then, only one person would be saved, and the rest of humankind would be damned.
(IV) Or if nobody would believe in the Resurrection of Christ, then not a single person would be saved, and all mankind should be damned.
But, in practice we do not expect the value of X to be extreme as in the case of (II) to (IV), but the value of X should lie somewhere between "All people saved" and "Nobody saved", according to Matthew 7:13, 14, 22:14, which was given to us by God's foreknowledge about future events and the aftermath of Judgement.
However, the real POINT here is that only God can tell you WHAT the Resurrection IS - Since, only God can be the Judge of this matter, but no earthly authority has any right to usurp his Throne, nor usurp the Judgement Seat of Christ.
PS. Whether you believe or not, if the Resurrection of Christ doesn't happen (by account of earthly authorities), then it would be equivalent to the situation of (IV), whereby, nobody at all believed the Resurrection of Christ - AND ALL MANKIND SHOULD BE DAMNED, ACCORDINGLY.
However, God would always honour his promise to respond in whichever manner is most appropriate - As contingent on the value of X, God will Judge (or decide) the actual numbers who are saved to enter the Kingdom of heaven - As each
one would be Judged according to his works = 2 Corinthians 5:10 and Revelation 20:13, but if all people are of the same opinion, then all of them would be treated the same, and treated as a "homogeneous group of like minded people", which is equal to X numbers of people.
Ultimately, I am counting on you not to choose the extreme values of X; namely, the would be situation of (IV), which would be tantamount to
suicide.