From The Collaborative International Dictionary of English v.0.48 :
Steadfastly \Stead"fast*ly\, adv.
In a steadfast manner; firmly.
[1913 Webster]
Steadfastly believe that whatever God has revealed is
infallibly true. --Wake.
[1913 Webster]
From WordNet (r) 3.0 (2006) :
steadfastly
adv 1: with resolute determination; "we firmly believed it";
"you must stand firm" [syn: firm, firmly,
steadfastly, unwaveringly]