Bindingly

adv. [f. prec. + -LY².] In a binding manner; so as to be obligatory.

1832.  Cheltenham Chron., 9 Feb., 2/1. Though the words are matters of course, the pledge appears to have been more bindingly given by the English than the French form of ratification.

1851.  G. S. Faber, Many Mansions (ed. 2), 10. With the Church of England, I maintain: nothing is, bindingly, to be received and believed as an Article of Faith, unless [etc.].