The essential mind (samadhi based on bodhicitta)
The realization of God rests upon two things, a powerful concentration (samadhi), and intellectual insight into the nature of Reality (vipashyana). Attainment of deepest samadhi is accompanied by mental and physical ecstasy, and enormous power of mind. When this powerful mind is turned towards a reasoned analysis of reality, enlightenment is near.
Importantly, in deepest samadhi the mind is fearless, and at last able to have faith in what it reasons, which at other times it is loathe to do. However, be warned that if one uses samadhi without the relentless will to strive for absolute perfection, at any cost, then great harm results. The ego will bask in the heavens of samadhi and become empowered thereby. Stagnation and contentedness will follow, and the potential for learning lost, not only in yourself, but in others also.
Therefore, samadhi is harmful to those not of high scope. It is safe only for those who, firstly, are in the human realm, and secondly, have accumulated enough reason to desire truth above all else - even above happiness. The mind of such a precious one is a "mind of enlightenment", called "bodhicitta". It is the mind that aspires to perfection, at all costs, and without compromise.
Without bodhicitta, and without complete faith in the ability of reason to carry one beyond this world, there will be continued striving for ego security. Without bodhicitta, no matter how great one's mental attainments and knowledge of Reality, a portion of the ego, the core, will remain fixed. Great happiness, even Nirvana, may ultimately be experienced, but the long-term consequences are horrible to contemplate, because each and every action of one who lacks bodhicitta is tainted with a solid and unchallenged faith in the very core of the ego.
One with bodhicitta may initially try to save himself, but reason soon gets the better of him, and will push him relentlessly onwards, into the arms of the Infinite.