Because that's what scripture teaches. And " it doesn't matter if they call themselves " Christian ". What matters is that they have done what scripture says is necessary:
"If you declare with your mouth, “Jesus is Lord,” and believe in your heart that God raised him from the dead, you will be saved."
It's interesting how you seem to turn this into a magical formula, that specific words must be uttered in order to engage the saving Grace of God. God will not operate properly unless the recipient utters in exact verbiage, "Jesus is Lord"? Do you believe this way about God?
Here's the thing, that passage is written to Christians, and it's not about specific ideas about God, but is simply referencing God in the language they understanding. It's not about speaking specific words or holding specific ideas in order for God to "let you into heaven", as you put it. The chapter is still speaking about faith and belief of the heart. And it is that type of belief, engaging the heart as opposed to just the conceptual mind, that saves.
Read what it says later on:
Consequently, faith comes from hearing the message, and the message is heard through the word of Christ.
But I ask: Did they not hear? Of course they did: "Their voice has gone out into all the earth, their words to the ends of the world."
Do you know what that is referencing? It's Psalm 19. Here's what Psalm 19 says about how everyone in the world hears that message.
The heavens declare the glory of God;
the skies proclaim the work of his hands.
2 Day after day they pour forth speech;
night after night they reveal knowledge.
3 They have no speech, they use no words;
no sound is heard from them.
4 Yet their voice goes out into all the earth,
their words to the ends of the world.
This is NOT ideas about God being preached that you have to acknowledge and confess for you to access God's Grace with the right words in order to be saved and let into heaven. That idea is foreign to these passages.
Romans is not disallowing those who understand God in different terms and ideas, yet do in fact respond to that one true God in their hearts and actions, which "confess" the truth of what is in their hearts. It's only the legalist who insists, 'no you have to say the right words first'. "By their fruits you shall know them", not by their doctrinal differences, as you see it.
BTW, even if we were to say only Christians have God's Love (something you believe and I do not), the point of this dialog originally was you saying that Christians who believe in reincarnation are not true Christians. Nothing whatsoever in this passage, or anywhere else in the Bible supports that view. Certainly not the 14th chapter of this same book you are quoting from.
There's no other way as Jesus himself said:
9 I am the door. If anyone enters by Me, he will be saved, and will go in and out and find pasture.
“Enter through the narrow gate. For wide is the gate and broad is the road that leads to destruction, and many enter through it. 14 But small is the gate and narrow the road that leads to life, and only a few find it.
Sure, Jesus is the Love of God personified. When he saying I am the door, that is true. That is Way that Jesus teaches, which is the Will of God, the Law of Love. That is the door you must go through. Love itself. That is the narrow gate, that few find, because they try to climb in some other way, such as the door of legalism, trying to earn the right for God to "let you into heaven".
Many will attempt to find an alternative route to God. They will try to get there through manmade rules and regulations, through false religion, or through self-effort... but there's no other door.
Exactly. Legalism, having the right beliefs and ideas as the keys to get "let into heaven" as your admission slip you earned through signing the contract at the church door in order to be saved, is an alternative route. It's climbing in through another way, which of course only lands you back where you started from.