Luke's understanding of Jesus – his Christology – is central to his theology. One approach to this is through the titles Luke gives to Jesus: these include, but are not limited to, Christ (Messiah), Lord, Son of God, and Son of Man.[31] Another is by reading Luke in the context of similar Greco-Roman divine saviour figures (Roman emperors are an example), references which would have made clear to Luke's readers that Jesus was the greatest of all saviours.[32] A third is to approach Luke through his use of the Old Testament, those passages from Jewish scripture which he cites to establish that Jesus is the promised Messiah.[33] While much of this is familiar, much also is missing: for example, Luke makes no clear reference to Christ's pre-existence or to the Christian's union with Christ, and makes relatively little reference to the concept of atonement: perhaps he felt no need to mention these ideas, or disagreed with them, or possibly he was simply unaware of them.[34]