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How can St. Justin say some in the past were Christians long before Christ?

Justin says also that the Divine Word is in all - not aspatial presence. He writes the law on hearts (Rom 2:15), i.e., makes known what morality requires. If a person obeyed that, he was following the Spirit of Christ, without knowing what he was following. And also, in view of Rom 8:9, he belonged to Christ, and so was a member of Christ = member of the Church, without external adherence of course.