大毘盧遮那成佛經疏 3.36

3.36

「於真言行善得決定」者，調於造立漫荼羅種種方便中，心得決定離諸疑網，所謂如是護身、如是結界、如是迎請、如是淨諸供具而以奉獻，以如是真言手印加持，乃至持誦進修，及成悉地時亦有無量次第法。如是備在下文及供養次第中，不能縷說。若行事時，及他決擇違妨，方復躊躇觀察，或取本尋撿，不名善作阿闍梨也。復次阿闍梨，以於瑜伽得決定故，隨有所作皆與三昧相應。如獻花時，即與花三昧相應，此中本尊明了現前。若奉香燈塗香、閼伽水等時，亦與香三昧乃至香水三昧相應，一一本尊亦隨事現前。如是一一緣中，皆是入法界門，皆見善知識，旋轉運用皆與理相應，不復臨事稽留方始作觀，當知是人堪作祕密阿闍梨也。

Regarding "well-established in the practice of mantras": it means that among the various upāyas of constructing a maṇḍala, the mind attains decisive certainty and is free from the net of doubts. That is to say, protecting the body in such a way, establishing the boundary in such a way, making invocations in such a way, purifying the offering utensils and presenting them in such a way, empowering with such mantras and hand seals (mudrās), and extending even to reciting and cultivating, as well as when accomplishing siddhi, there are also immeasurable sequential procedures. These are fully set forth below and in the ritual manuals of offering, and cannot be recounted in detail here. If, when performing the rites or when others seek to resolve obstacles and contradictions, one then hesitates and ponders, or consults manuscripts to look them up, one is not called an adept ācārya. Furthermore, because the ācārya has attained decisive certainty in yoga, whatever he performs accords with samādhi. For instance, when offering flowers, he immediately accords with the flower-samādhi, and the tutelary deity therein manifests clearly before him. When presenting incense, lamps, scented unguent, argha water, and so forth, he also accords with the incense-samādhi up to the perfumed-water-samādhi, and each tutelary deity also manifests in accordance with the rite. In every such condition, it is an entrance into the dharmadhātu, and in each he sees a spiritual friend (kalyāṇamitra); his movements and operations all accord with the principle, and he no longer hesitates only to begin visualization when facing the occasion. You should know that such a person is qualified to be an esoteric ācārya.