"before the L-rd": They think they are standing before flesh and blood — they are standing before the L-rd!
"before the Cohanim and the judges who shall be in those days": This is as R. Yossi Haglili said: Would it enter your mind that they (should stand before) a judge who is not in your days! (The reference is) rather, to (a judge) who had been kin (to one of the litigants [e.g., married to his sister, in which instance he would not be fit for judging his case]), and who had become "distant" (by her death — his present status [thus, "in your days"] permitting him to judge the case.) Variantly: (The reference is) to one who was certified (to judge you) in those days, in keeping with (Koheleth 7:10) "Do not say: What has happened that the former days were better than these, etc."