A few of questions:
1. In order to get into heaven is it absolutely necessary to be a Christian?
It is absolutely necessary to accept Christ as your Saviour and Lord.
John 14:6 (KJV)
6 Jesus said unto him, I am the way, the truth, and the life: no man comes unto the Father, but by me.
Acts 4:12 (NKJV)
12 Nor is there salvation in any other, for there is no other name under heaven given among men by which we must be saved."
1 John 5:11-12 (NKJV)
11 And this is the testimony: that God has given us eternal life, and this life is in His Son.
12 He who has the Son has life; he who does not have the Son of God does not have life.
John 3:36 (NKJV)
36 He who believes in the Son has everlasting life; and he who does not believe the Son shall not see life, but the wrath of God abides on him."
2. If so, then do all others end up in hell?
Yes.
Matthew 10:28 (KJV)
28 And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in hell.
Matthew 18:9 (NKJV)
9 And if your eye causes you to sin, pluck it out and cast
it from you. It is better for you to enter into life with one eye, rather than having two eyes, to be cast into hell fire.
Mark 9:43-44 (NKJV)
43 If your hand causes you to sin, cut it off. It is better for you to enter into life maimed, rather than having two hands, to go to hell, into the fire that shall never be quenched--
44 where
'Their worm does not die, And the fire is not quenched.'
2 Peter 2
Matthew 25:46 (KJV)
46 And these shall go away into everlasting punishment: but the righteous into life eternal.
Matthew 7:13 (KJV)
13 Enter you in at the strait gate: for wide
is the gate, and broad
is the way, that leads to destruction, and many there be which go in thereat:
Romans 2:5-9 (NKJV)
5 But in accordance with your hardness and your impenitent heart you are treasuring up for yourself wrath in the day of wrath and revelation of the righteous judgment of God,
6 who
"will render to each one according to his deeds":
7 eternal life to those who by patient continuance in doing good seek for glory, honor, and immortality;
8 but to those who are self-seeking and do not obey the truth, but obey unrighteousness--indignation and wrath,
9 tribulation and anguish, on every soul of man who does evil, of the Jew first and also of the Greek;
3. If not then what becomes of them?
There is no "if not."
4. What is the evidence for your answer?
See above.
Peace.