Ruuth was ordained in the Växjö diocese of the Church of Sweden in 1953 after studying Protestant theology in Lund. After ten years of parish service in Argentina, he became pastor in Härnösand in 1963. In 1966, he returned to Argentina as a theology professor at the university in José C. Paz (near Buenos Aires), became director of the church study center (stiftsgård) Söråker in 1971, provost in Örnsköldsvik in 1974, and director of the Swedish Luther Aid in 1985. In 1995, Ruuth received his doctorate in theology with a thesis on the Latin American Pentecostal church Igreja Universal do Reino de Deus.

In Argentina, Ruuth wrote and composed the blessing song "La paz del señor" in 1968, which became known through the World Day of Prayer in 1988. Eugen Eckert added the German text "Bewahre uns, Gott" to this song in 1985.[2] This song is found in numerous songbooks and has often been adapted.