The answer may not be what you think, and those who qualify may not be whom you might expect to qualify...
Those who are "friends of the Lord," are, in the Lord's own words, those who hear, understand, and obey His voice spoken into their hearts (spirit-mind connection) by the Spirit of truth. What this produces, then, is the correct "rhema" understanding of the truth, covenant promises, and commands of the Lord resulting in the manifestation of acts of faith. Thus, it may be helpful to be trained in Hebrew and Greek, but no amount of intellectual training apart from "hearing" and "seeing" by the Spirit of truth will produce faith in our thoughts or our actions.
It is, then, those who "hear" His truth, His covenant promises, and His commands" and, in intimate obedience, do what they hear by faith, His "bondservants" and "friends."