Luke 12:1

In the mean time
While Christ was discoursing with the Pharisees, and they were using him in the vilest manner, throwing out their invectives against him in order to draw off the people from him:

when there were gathered together an innumerable multitude of
people.
There were "myriads" of them, as in the original text, and a myriad is ten thousand; the meaning is, that there were several thousands of them:

insomuch that they trod one upon another;
striving to get near to Christ, either to see his person, or to hear his discourses; and particularly, what he would say to the Pharisees, who had fallen upon him in so violent a manner:

he began to say unto his disciples first of all;
he directed his discourse not to the Pharisees, nor to the multitude, but to his disciples in the first place; at least, chiefly to them; for whom he had a regard, who were his dear friends, and were to be the preachers of his Gospel every where; and therefore it was proper that they should be aware of the dissembling arts of the Scribes and Pharisees, and have their minds fortified against approaching dangers, persecutions, and death itself: the last phrase, "first of all", is omitted in the Vulgate Latin version; and by all the Oriental versions, it is joined to the next clause, and read thus, "especially", or

before all things, beware of the leaven of the Pharisees, which
is hypocrisy;
expressed both in their doctrines, and in their lives; which carried a great show of piety and holiness, but was in appearance only: very aptly is hypocrisy in doctrine and manners, compared to leaven; which at first is small and little, but gradually increases and spreads itself, and lies hid and covered, and is not easily discerned, nor its influence and effects observed; but in time, it infects and corrupts the whole of men's principles and practices, and puffs and swells them up with a vain opinion of themselves; and when our Lord bids his disciples beware of it, his meaning not only is, that they take heed that they were not infected with it themselves, but that they were not imposed upon by the specious pretences of these artful and designing men.