From 1947 to 1953 Szlaga attended elementary school in Gdynia and then went attended Collegium Leoninum in Wejherowo and Collegium Marianum in Pelplin. He studied at the Pelplin Higher Seminary ( Wyższe Seminarium Duchowne w Pelplinie) and was ordained a Roman Catholic priest in Chelmno (Culma, Kulm) in 1963.
From 1965 to 1969 Father Szlaga continued his biblical studies at the Catholic University of Lublin and from 1972 to 1973 at the Pontifical Biblical Institute in Rome. Upon completing his doctorate in theology, Szlaga took on academic duties in addition to his priestly duties:
Assistant to professor of biblical studies, Catholic University of Lublin
Vice-dean at Catholic University of Lublin
Dean of the Faculty of Theology 1981-1984
Lecturer at the Major Seminary in Lublin, Pelplin Seminary and University of Torun. Professor at the University of Gdańsk"s Faculty of Languages and History 1991-2001
In 1988, Father Slaga was named Auxiliary Bishop of Chelmno and Titular Bishop of Mascula.
In 1992 he was conscrated as Bishop of Pelplin.
Szlaga"s health declined just before he was to act as main consecrator of the new Auxiliary Bishop Wiesław Śmigiel and he died on April 25, 2012. He is buried beside the altar at the Cathedral Basilica of the Assumption of the Blessed Virgin Mary at Pelplin Abbey.
Member of the Studiorum Novi Testamenti Societas
ordinary member of the Scientific Society of CUL.