Colson's essentials and his case rests on his interpretation of Jesus' apparent claim in John 14:6 that no one goes to God but through Christ. There are some other biblical passages that seem to make the same claim, notably John 3:16. But, inevitably, Colson has to cherry-pick passages in order to make his point. There is no dissertation in the Bible saying that only those who believe the things he believes are saved. The problem with cherry picking is, of course, that one can use it to prove most anything in the Bible that one wants to including the "fact" that the Bible supports universal salvation. Luke 3:6, as one example of several such passages, quotes John the Baptist quoting Isaiah that when the paths of the Lord are made strait in Christ "All mankind will see God's salvation." (TEV) It's clear, it's in the Bible: all humanity will be saved. Now, Colson would say that this is a misuse of the Bible, and he would be right. It cherry-picks a passage to prove a position while ignoring the complexities of the passage itself, which is precisely the problem with Colson's first, foundational point. It is based on a cherry-picked passage and ignores the complexities of context and interpretation of the passage.
A better approach is to start with Christ as the four gospels portray him and work from there. On that basis, we can make a case for universal salvation. Christ proclaimed repeatedly that when the Kingdom comes it will be sinners, lepers, tax collectors, prostitutes, and the like that will sit at his table. The orthodox of Jesus' day—the wealthy, educated, and pious elite—believed these folks to be impious, sinful people who were being punished by God, but Christ disagreed. The very people that the orthodox believed were damned were those Christ said will sit at his table in the Kingdom. Indeed, Colson's judgmental and, frankly, arrogant tone sounds suspiciously like the judgmental, arrogant Pharisees and Sadducees who were at tragic odds with Christ.
It is because of Christ (as well as the requirements of justice pure and simple), as I have written before in these postings, that we ordain women in the Presbyterian Church (USA) in spite of clear scriptural warrant against doing so. Cherry-picking biblical literalists have cherry-picked reasons to oppose the ordination of women. The example of Christ leads us, however, to reject unjust and unloving interpretations of Scripture even when its words can be used to make the case for unjust and unloving conclusions.
Colson is correct that, "Jesus Himself was very clear." He was clear by the way he lived and in the whole body of his teachings. He was clearly on the side of those that the pious orthodox folks judged as enemies of God. Jesus clearly saw that those whom the orthodox condemned were, in fact, his chosen people. This is to say that Colson's basic assumption that Jesus damns "unbelievers" to eternal hellfire does not stand when viewed from the whole person of Jesus Christ, his works, his message, and his ministry. It is not orthodoxy that matters. It is Christ who does.