The author rereads the Instruction Inter Oecumenici of 26 September 1964, seeking to offer the reader four keys to approaching this text on the 60th anniversary of its promulgation. First of all, in dialogue with Sacrosanctum Concilium, he highlights (I) the theological sense of the liturgy that emerges from the text, then he considers (II) the theological sense of participation, then (III) formation as progressive participation in the paschal mystery and, finally, (IV) the liturgical nature of ecclesial action.