Respectfully, I don't see how Christianity helps. It wants you to sacrifice yourself and your happiness on Earth for the sake of a supernatural promise of eternal life and joy in heaven. The trouble is, we only live once, and the only happiness is that which we can find in reality. The supernatural is so called because no one has ever shown that it exists, and indeed, it could not exist without violating many known laws of nature.

As others have noted, rational self interest means seeking your happiness and survival with a view to the full term of your life and the long term relating to all your goals and values. It is not short-term grasping, it is long-term and thus relies on well-established principles.

study RATIONAL self-interest....

Self interest is always present. Rational self interest is not always present.
From The Objectivist's Ethics.
"A being who does not know automatically what is true or false, cannot know automatically what is right or wrong, what is good for him or evil. Yet he needs that knowledge in order to live. He is not exempt from the laws of reality, he is a specific organism of a specific nature that requires specific actions to sustain his life. He cannot achieve his survival by arbitrary means nor by random motions nor by blind urges nor by chance nor by whim. That which his survival requires is set by his nature and is not open to his choice. What is open to his choice is only whether he will discover it or not, whether he will choose the right goals and values or not. He is free to make the wrong choice, but not free to succeed with it. He is free to evade reality, he is free to unfocus his mind and stumble blindly down any road he pleases, but not free to avoid the abyss he refuses to see. Knowledge, for any conscious organism, is the means of survival; to a living consciousness, every “is” implies an “ought.” Man is free to choose not to be conscious, but not free to escape the penalty of unconsciousness: destruction. Man is the only living species that has the power to act as his own destroyer — and that is the way he has acted through most of his history."

My take is Ayn Rand draws a difference between rational self interest (Virtue of Selfishness) and flat out greed as they are not the same thing, unlike the left that tries to equate the two.

Man—every man—is an end in himself, not a means to the ends of others; he must live for his own sake, neither sacrificing himself to others nor sacrificing others to himself; he must work for his rational self-interest, with the achievement of his own happiness as the highest moral purpose of his life. Ayn Rand

"Rightful liberty is unobstructed action according to our will within limits drawn around us by the equal rights of others. I do not add 'within the limits of the law' because law is often but the tyrant's will, and always so when it violates the rights of the individual." Thomas Jefferson