VATICAN CITY — From his stylish living room overlooking St. Peter’s Square, Cardinal Walter Kasper doesn’t come off as a figure at the center of one of the greatest storms swirling in Catholicism in decades.

Relaxed on a black leather sofa, the German theologian says he fully expected the knives would come out when, at Pope Francis’ request, he made a suggestion that challenged a deep church taboo and has dominated debate ahead of a landmark meeting on Catholic family life that opens this weekend. The issue is not abortion, contraception or gay marriage. It is the fate of Catholics who divorce — and the outcome will be a key test of how far Francis’ reform agenda will go.

Delivering a speech to a closed meeting of cardinals last February, Kasper suggested that Catholics who remarry without an annulment, a church declaration that the first marriage was invalid and thus never existed, might receive Communion after a period of penance.