Who can act as our spiritual master?

Only that great personality who has been sent by the Lord from the spiritual
world into this world to deliver fallen souls like us, who delivers people
who are afflicted with the threefold miseries and sends them back to
Godhead, who is most dear to the Supreme Lord and who represents Him, and
who is the messenger of transcendental subject matter, is capable of acting
as our spiritual master. He is an actual devotee and an actual spiritual
master whose ax-like words remain always sharpened to sacrifice our
propensities for material enjoyment and dry renunciation.
A spiritual master is that person who has no concept, duty, or business
other than Krsna's service, and who understand that Krsna is the only object
of service. He is not eager to hear anyone's flattery and he is a brave
preacher of the Absolute Truth.

One who does not speak anything other than hari-katha, who does not instruct
others to do anything but serve Hari, and who does not himself engage even
for a moment in any activity other than such service, is qualified to be a
spiritual master. An insincere hypocrite cannot be guru. One who aspires for
mundane activities cannot become a spiritual master. Pseudo spiritual
masters should be exposed. If a spiritual master utilizes the ingredients
collected by his disciples-the wealth, alms, etc.-for his own enjoyment or
simply to accumulate gold, women, and fame, his disciples should totally
reject such a spiritual master, knowing him to be a cheater. We should not
hear anything from such dishonest persons. A person who usurps the
ingredients meant for Krsna's service can never be called a spiritual
master.

The Naradiya Puråna states, iha yasya harer dasye, karmana manasa
gira/ nikhilasv apy avasthasu, jivan-mukta? sa ucyate: "A person acting in
Krsna consciousness with his body, mind, intelligence, and words, is a
liberated person even within the material world, although he may be engaged
in many so-called material activities."
We need not associate with the atheists who have come to engage in social
service rather than Krsna's service. Such people can benefit neither
themselves nor others. While performing social service, they fall into
maya's pit and make others fall into that pit also. We do not associate with
those who make a show of chanting the holy names on beads just to cheat the
Lord, or who raise a great hue and cry yet do not see Krsna in every sound
vibration and Gaurasundara in every utterance. The goal of all knowledge is
to develop our relationship with Krsna.

If we develop a propensity to serve
the Lord under the spiritual master's order, in the association of devotees,
we will learn to see the entire world as made for the Lord's service. We
will then utilize everything we see for that service and thus attain
auspiciousness.
If we associate and serve that great personality who sees the Lord
everywhere, who sees everything in relation to the Lord, who sees his
spiritual master everywhere, who thinks himself humbler than a blade of
grass, who is more tolerant than a tree, who does not desire respect for
himself, who is always ready to respect others, and who in this way
constantly chants the Lord's holy name, then the path to perfection will
open. Simply out of good fortune, we obtain such a bona fide spiritual
master.

We cannot reach Gaurasundara's shelter by accepting a servant of
Maya as our spiritual master and thus maintaining our enjoying spirit.
Although Sri Gaurasundara may not currently be present in this world, if we
sincerely associate with guru and the Vaisnavas, dovetailing our mentality
with the mentality of such Vaisnavas-if we mix our desire with their
desire-then we will, by the mercy of Sri Gaurasundara, certainly attain
auspiciousness. If we can take complete shelter at the lotus feet of a
spiritual master who is conversant with the science of Krsna, then by his
proper association, service, and subordination we will attain ultimate
benefit.

(From Srila Bhaktisiddhanta "Amrta Vani", an upcoming book)