If one would recite contrary to the science of mantras;

the faithless will not achieve their wishes and needs,

Even the faithful will be long delayed in accomplishing siddhi.

- Sakya Pandita or Kunga Gylatshan Pal Zangpo

The science of mantras has been explored by various Buddhists and the nAstika Sakya Pandita's advice is valuable even to us Astika upAsakas.

- If one recites mantras incorrectly i.e.

a. without an initiation

b. without the right pronunciation

c. without the observance of necessary conditions of chitta (mind), desha (space or location) and kAla (time)

d. Intent or samkalpa

AND without faith (can roughly be described as bhakti towards the iShTa and guru), siddhi will not be accomplished.

- If there is Faith (or bhakti) but the mantra is recited incorrectly (i.e. without fulfilling the above four conditions), the attainment of Siddhi is possible but is clouded by long delays, lasting up to several lifetimes. In such cases, it is generally seen that pure faith based practices eventually lead in one lifetime or another to skillful means of correct upAsanA, the two combining eventually to result in siddhi.

The case of complete adherence to the mantra shAstra but lacking bhakti or faith (in terms of the nAstika tantras) is rhetorical as neither the Hindu nor bauddha tantras accept entry into mantrayAna when the aspirant lacks faith, bhakti, vishvAsa etc. Needless to say, these concepts are just that - concepts - at the beginning of practice and are strengthened through the skillful practice of mantrayAna.

At the cost of exhibiting favoritism, I cannot help but point out how lucidly the same is explained by shrI bhAskararAya while explaining the sUtra, bhaktimAnupAsIta. Thus, skillful means plays as important a role as faith or bhakti in mantra shAstra, if not more.