Like Christians, Muslims believe that the present life is only a trial preparation for the next realm of existence. This life is a test for each individual for the life after death. A day will come when the whole universe will be destroyed and the dead will be resurrected for judgment by God. This day will be the beginning of a life that will never end. This day is the Day of Judgment.
On that day, all people will be rewarded by God according to their beliefs and deeds. Those who die while believing that "There is no true god but God, and Muhammad is the Messenger (Prophet) of God" and are Muslim will be rewarded on that day and will be admitted to Paradise forever, as God has said:
"And those who believe and do good deeds, they are dwellers of Paradise, they dwell therein forever." The Holy Qur'an, Chapter 2, Verse 82
But those who die while not believing that "There is no true god but God, and Muhammad is the Messenger (Prophet) of God" or are not Muslim will loose Paradise forever and will be sent to Hellfire, as God has said:
"And whoever seeks a religion other than Islam, it will not be accepted from him and he will be one of the losers in the Hereafter." The Holy Qur'an, Chapter 3, Verse 85
And as He has said:
"Those who have disbelieved and died in disbelief, the earth full of gold would not be aceepted from any of them if it were offered as a ransom. They will have a painful punishment, and they will have no helpers." The Holy Qur'an, Chapter 3, Verse 91
One may ask,'I think Islam is a good religion, but if I were to convert to Islam, my family, friends, and other people would persecute me and make fun of me. So if I do not convert to Islam, will I enter Paradise and be saved from Hellfire?' The answer is what God has said in the preceeding verse,"And whoever seeks a religion other than Islam, it will not be accepted from him and he will be one of the losers in the Hereafter."
After having sent the Prophet Muhammad (pbuh) to call people to Islam, God does not accept adherence to any way of life other than Islam. God is our Creator and Sustainer. He created for us whatever is in the earth. All the blessings and good things we have are from Him. So after all this, when someone rejects belief in God, His Prophet Muhammad(pbuh), or His religion of Islam, it is just that he or she be punished in the Hereafter. Actually, the main purpose of our creation is to worship God alone and to obey Him, as God has said in the Holy Qur'an, Chapter 51, Verse 56.
This life we live today is a very short life. The unbelievers on the Day of Judgment will think that the life they lived on earth was only a day or part of a day, as God has said:
"He (God) will say, "How many years did you stay on the earth?" They will say:"We stayed a day or part of a day...." The Holy Qur'an, Chapter 23, Verses 112-113
And He has said:
"Did you then think that We created you in jest (without any purpose), and that you would not be returned to Us (in the Hereafter)? So, God is exallted, the True King. None has the right to be worshipped but Him..." The Holy Qur'an, Chapter 23, Verses 115-116
In comparing this world with the Hereafter, the Prophet Muhammad(pbuh) said:
The value of this world compared to that of the Hereafter is like what your finger brings from the sea when you put it in and then take it out. Narrated in Sahih Muslim, #2858 and Musnad Ahmad, #17560
The meaning is that, the value of this world compared to that of the Hereafter is like a few drops of water compared to the sea.
A Brief Illustrated Guide to Understanding Islam