Paul best describes the criteria for salvation in Romans 10, which explains that by believing within the heart, and confessing with the mouth (or in some form of physical acknowledgment), you will be saved.
It becomes a matter of the heart. Faith in the Son of God and the finished work at Calvary, is required, but only God can distinguish between those who have a genuine belief, from those who simply have a form of Godliness.
Religious pluralism is one religion which is gaining ground in our time. This is the religion which believes that there are many paths to heaven. President Obama practices this religion, unless his views have changed since his 2004 interview with Catherine Falsani, a reporter for the Chicago Sun Times.
There is also at least one denomination within our own faith, who seems to complete the requirements for salvation, but, whose doctrine, in my opinion, contains dangerous errors, such as participatiing in the practice of the extreme veneration of Mary, where she is allowed to serve as a 2nd Mediator, with some even considering her the Queen of Heaven.
There are also those who have completed the requirements for salvation, but are lukewarm in their faith, which according to Revelation 3:16, will cause Jesus to reject them. I believe this lukewarmness to mean, those who have been practicing both Christianity and wickedness, as a lifestyle, and those who are ashamed to boldly acknowledge their faith in Jesus, before others.
At the end of life, God knows those who are His, and so, those who do not have the Son, will not have eternal life in Heaven, and for those who have lived their lives trying to enter Heaven by some way, other than through the gate, God will determine what they've accomplished by doing so.
John 10:7
Therefore Jesus said again, “I tell you the truth, I am the gate for the sheep. All who ever came before me were thieves and robbers, but the sheep did not listen to them. I am the gate; whoever enters through me will be saved.