This article is a general description of the concept of Heaven as it pertains to Christianity. Many of Harry Turtledove's Christian characters subscribe or have subscribed to the concepts described below, depending on their denomination.
Historically, Christianity has taught Heaven as a generalized concept, a place of eternal life, in that it is a shared plane to be attained by all the elect (rather than an abstract experience related to individual concepts of the ideal).
Specifics of this belief do vary, depending the denomination of Christianity in question.
Roman Catholics believe that entering Purgatory after death (physical rather than ego death) cleanses one of sin (period of suffering until one's nature is perfected), which makes one acceptable to enter heaven. In the Eastern and Oriental Orthodox Churches, it is only God who has the final say on who enters heaven.
In Protestantism, eternal life depends upon the sinner receiving God's grace (unearned and undeserved blessing stemming from God's love) through faith in Jesus' death for their sins, his resurrection as the Christ, and accepting his Lordship (authority and guidance) over their lives.