BHAKTI know it , understand it. Explained in details by Swami Vivekananda

                                                        DEFINITION OF BHAKTI

Bhakti-Yoga is a real, genuine search after the Lord, a search beginning, continuing, and ending in love. One single moment of the madness of extreme love to God brings us eternal freedom. "Bhakti", says Nârada in his explanation of the Bhakti-aphorisms, "is intense love to God"; "When a man gets it, he loves all, hates none; he becomes satisfied for ever"; "This love cannot be reduced to any earthly benefit", because so long as worldly desires last, that kind of love does not come; "Bhakti is greater than karma, greater than Yoga, because these are intended for an object in view, while Bhakti is its own fruition, its own means and its own end."

The one great advantage of Bhakti is that it is the easiest and the most natural way to reach the great divine end in view; its great disadvantage is that in its lower forms it oftentimes degenerates into hideous fanaticism. The fanatical crew in Hinduism, or Mohammedanism, or Christianity, have always been almost exclusively recruited from these worshippers on the lower planes of Bhakti. That singleness of attachment (Nishthâ) to a loved object, without which no genuine love can grow, is very often also the cause of the denunciation of everything else.

 All the weak and undeveloped minds in every religion or country have only one way of loving their own ideal, i.e. by not considering the other ideals at all.

This kind of love is somewhat like the canine instinct of guarding the master's property from intrusion; only, the instinct of the dog is better than the reason of man, for the dog never mistakes its master for an enemy in whatever dress he may come before it. Again, the fanatic loses all power of judgment. Personal considerations are in his case of such absorbing interest that to him it is no question at all what a man says — whether it is right or wrong; but the one thing he is always particularly careful to know is who says it. The same man who is kind, good, honest, and loving to people of his own opinion, will not hesitate to do the vilest deeds when they are directed against persons beyond the pale of his own religious brotherhood.

Danger exists only in that stage of Bhakti which is called the preparatory (Gauni). When Bhakti has become ripe and has passed into that form which is called the supreme (Parâ) where The bhakti evolves, refines, and purifies to extreme level.

Swami Vivekananda

Now how we will know that our love is purified enough?, how to purify? who will give guidance if stuck?

A GURU is mandatory