him - dative of possession; in - the OE preposition "on" becomes confused with, and on the whole takes over the functions of ,"in". There is a general preference for "on" in OE as there is in this text. This is the only place in the poem where "in" is used. It may be intended to emphasise the meaning here. There certainly seems a play between the idea of carrying a relic or an image of theCross on one's breast and carrying the true meaning of the "Cross" in one's breast/heart. breostum - for the plural cf. the note to 63b "heafdum".