Numa omnium primum deorum metum Romanis iniciendum esse ratus est. Qui cum descendere ad animos non posset sine aliquo divino miraculo, simulat sibi cum dea Egeria nocturnos sermones esse: 'eius monitu se sacra quae dis acceptissima essent instituere, et sacerdotes suos [ : proprios] cuique deorum praeficere.'
I'm a little confused about some of the above.
I'm taking it that 'omnium primum' means that Numan was the first of all (the Kings?) and then we have 'ratus est' - thought - and then metum....iniciendum esse - gerundive - translated probably as an English particple or infinitive. So I would translate the first part of this as follows:
Numa was the first of all to think of instilling fear of the gods into (Romanis is dative, right?) the Romans.
'descendere' here I take to mean 'penetrate' or 'move'. So I would continue: 'He could not, with out some divine miracle, move the spirits [of the people] he pretended [is Livy really so candid?] to have had nightly conversations with the goddess Egeria: 'by her warning he himself should institute fitting sacrifices to the gods [sacrifices that would be fitting - pluperfect subjunctive] and appoint appropriate / special priests to her of (in preference to / above the other?) the gods'.