ISB ~ Matthew 5 v31

a certificate of divorce:Divorce and remarriage were permitted under the Old Covenant only because of Israel’s sinfulness (19:8; cf. Deut 24:1-4). In the New Covenant, remarriage leads to adultery (CCC 2382).

except on the ground of unchastity:Matthew alone records this added “exception clause” (cf. 19:9). For the meaning of this clause, see topical essay: Jesus on Marriage and Divorce at Mt 19.