One of Christianity's great truths is that God has come to be with us, as "Emmanuel, God with us." God comes to "make his home in us" (John 14). Therefore, we are never alone.
Dallas Willard writes: "God is able to penetrate and intertwine himself within the fibers of the human self in such a way that those who are enveloped in his loving companionship will never be alone." (Willard, Hearing God: Developing a Conversational Relationship with God, p. 59).
This is life's greatest thing; viz., to live in the presence of God. Conversely, life's most devastating thing is to dwell outside of the fortress of God. Willard writes:
"The fact that only God can take away our aloneness by his presence explains why the ultimate suffering and punishment is separation from the presence of God. The psalmist cries out in terror, “Do not cast me away from your presence, and do not take your holy spirit from me” (Ps 51:11)." (Ib.)