The Sanskrit term ‘Asthi’ means ‘to accept the existence’ and ‘Nasthi’ means, denying the existence. These form the roots for the words ‘Asthikam’ and ‘Nastikam’ which translate as theism and atheism respectively.
An Asthika believes in the existence of God. Acts like going to Sabari Mala every year, attending an Ati Rudram, performing Chandi Homam, propitiating the Navagrahas, going on pilgrimages, reading the Srimad Bhagavatam and Srimad Ramayana, having darshan of Sadhus and saints – these are typical of an Asthika.

A ‘Bhakta’ (devotee) is a step higher and he sticks on to one and only one God. Hanuman, Azhwars, Meera, Saint Thiyagaraja etc. are supreme Bhaktas. One cannot find a single Pathigam in Devaram in praise of Lord Vishnu, neither can one find a single Pasuram in Divya Prabandham that sings Lord Siva; Hanuman knows not Krishna, Gopis know not Rama.
At the same time, a Bhakta never detests or talks ill of another deity. He respects all other forms just like a wife has true love and affection for her husband and yet respects all his relatives.

In this context, one might get a doubt if the one-pointed devotion to a single deity speaks of the narrow mindedness of the person.
A true wife being faithful to her husband is indeed a virtue. It is wrong to term it as narrow mindedness, isn’t it?
In fact, one-pointed devotion is very rare virtue that comes by spontaneously.

Then, almost automatically comes the question of why we have so many deities in our religion. The simple answer is that, human tastes are widely varied and the numerous deities of our religion give a seeker the freedom to choose from these different God-forms, a form that best suits his taste and mental disposition.

In short, the truth is one and not many and all paths lead to one, same Truth.
Just like our mind oscillates in worldly affairs, it tends to waver in spiritual matters as well.
In order to obtain water from the earth, it is a smart move to keep digging constantly at the same spot. Our efforts would prove futile if they were directed at digging multiple different places.
So, direct your energy in one direction, towards one God, through one Master, and you will reach the destination quicker.