What Exactly Must A Non-Christian Do To Attain Eternal Life?

I am a Hindu.First we look differently at Eternal Life.Like according to Hindu belief system, Heaven and Hell are there, but are not places one must covet to get in. For Heaven in Hindu Belief is only a place of peace prosperity and happiness(No Pain, disappointment etc. so much so that you wish and it will happen). Yet a Hindu is encouraged to desist from going to Heaven.According to the Hindu Religion one is asked to strive for Liberation from the Cycle of Births and Deaths for fundamental to Hindu Religion is a belief in Rebirth and perfection to be "like God" is attained over several lifetimes in this world and others. Thus life in Heaven in short is not an Eternal Life.Heaven is the result of good deeds done here on earth. One of the important Good Deeds is Charity for the deserving. Thus eternal life not in a physical body, but is in three possible forms. To be with God in His abode but distinct from Him. To be a part of God but still distinct from Him. To merge into God in an inseparable manner. Thus in each of these forms of Liberation (called Moksha) you have no destruction, decay or disease. Now how to attain this form of "Eternal Life", if you could call it so? In the first you be devoted to God so dearly by not wanting to miss Him for even a second, and He acknowledges your desire to be with Him and grants liberation(Moksha = Freedom from having to be born anywhere, undergo suffering of anykind and the like). This is the so called attaining Moksha by the Grace of God. This is the "description" of Moksha acceptable to Dualists among Hindus. Then there is the second one wherein you come to be one of God- by surrendering your will to Him and living a life of righteousness and deep devotion to Him. Here too God takes one into His Abode. There is a slight difference between, the Dualist view and the Qualified Monist view, as the second one is called. In the First One liberation is by God's Grace alone(what we do or not do is immaterial). The second one is placing our faith totally in Him in surrender.Thereon you believe that He will never forsake us. The last one is of one's merger into God. This kind of Moksha is possible, by "knowing" Him in troth. Now, this knowing is different from knowing how to get to a place in the real world, or knowing the Laws of Nature through eperiment and reasoning, and knowing that God is like so through Scriptural Texts. This is by far the toughest form of getting to God.In full knowledge, for one thing, you will take whatever he metes out to you, good or bad, without complaint and ideally cheerfully. It also means that you must have made your Ego Zero. IOW holding in thought mind and body that He alone exists and even my own existence is but a shadow of His, is the clue to moksha by merger. There is yet one more kind that is still difficult to explain. Here one has to "know" that one is None othar than THAT ONE. This is the Non Dual moksha! Here God is not seen as a Creator, Sustainer and Dissolver of His Creation but as the ONE nad ONLY enduring reality- the substratum of all experience. Thus in Hinduism there are several paths to God and one is free to choose. It is not right to call the Hindu way of looking at the relationship with God to seek something like Eternal Life. In every form of Hinduism the "soul" is indestructible(=deathless) and eternal!

I can not speak foe the other religions you mentioned but, as a Buddhist I can give you a brief summation of Buddhist beliefs regarding eternal life. The teachings of the Buddha do not discuss eternal life and in fact recommend against spending time concerning yourself with it. The same applies with the concept of reincarnation. Buddhism is all about improving this life for others and yourself. It discourages useless speculation about what lies beyond.