The thesis studies the Quaker thought of Robert Barclay (1648-90), focusing upon his theological views on the relationship between self and others, from his soteriology to peace testimony. The thesis has two main objects; the first is to raise the question about the modern view of Barclay. His theology has long been negatively treated as an exterior or foreign factor by the present Liberal Quakers' self-affirmative theology. The second is to explore another possibility of understanding Quakerism and its practical applications in pacifism, from a different viewpoint than the empirical, individualistic one today. The whole research is conducted by using the concept of self as the central axis for analysis. By analysing Barclay's theology and his peace testimony, and placing them within the contexts of traditional Christianity, the thesis indicates the other-absent character of Liberal Quakerism, and it shows an old-new Christian task that Quakers take on as a testimony to God and to Christ's openness towards others. The specific themes are (1): 'self-denial' in Barclay's theology as a counter-faith against self-reductive orthodox Calvinism and Arminianism, (2): Peculiarity of Liberal Quakerism and their historiography endorsed by an unexamined premise of self-affirmation, and the re-estimation of Barclay from that viewpoint, (3): Liberals' self-centred orientation in pacifism in their simple belief in the calculability and reducibility of others, (4): The double-stance, or the possibility and impossibility, in Barclay's view on perfection and the Kingdom, which is the reflection of self-other relations, as well described in Barclay's theoretical connection of perfection to Christ's command to 'love one's enemies,' (5): the Church as a place to embody the Kingdom, and its practical extension to the entire world in pacifism, for the realisation of the Godly rule beyond the self-contained logic.