The Collaborative International Dictionary of English v.0.48:

Diligently \Dil"i*gent*ly\, adv.
   In a diligent manner; not carelessly; not negligently; with
   industry or assiduity.
   [1913 Webster]

         Ye diligently keep commandments of the Lord your God.
                                                  --Deut. vi.
                                                  17.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diligently
    adv 1: with diligence; in a diligent manner; "we may diligently
           observe the Lord's supper on the first day of the week,
           diligently preach the gospel, or minister to the saint"

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "diligently":
   ardently, assiduously, carefully, cautiously, circumspectly,
   considerately, devotedly, doggedly, energetically, faithfully,
   fervently, heedfully, indefatigably, industriously, insistently,
   laboriously, lovingly, loyally, mindfully, painstakingly,
   patiently, perseveringly, persistently, persistingly,
   pertinaciously, ploddingly, regardfully, relentlessly, resolutely,
   sedulously, sloggingly, solicitously, strenuously, tenaciously,
   tenderly, thoroughgoingly, thoroughly, thoughtfully, tirelessly,
   unabatingly, unfalteringly, unflaggingly, unflinchingly,
   unintermittingly, uninterruptedly, unrelentingly, unremittingly,
   unrestingly, unsleepingly, unsparingly, unswervingly, untiringly,
   unwaveringly, unwearyingly, vehemently, wearilessly, with care,
   with great care, zealously