Give not that which is holy unto the dogs, neither cast ye your pearls before swine,

MATTHEW 7:6

Matthew 7:6  Give NOT that which is HOLY unto the DOGS, neither cast ye your PEARLS before SWINE, lest they TRAMPLE them under their feet, and turn again and REND YOU. Give not that which is holy to the dogs,.... 

Dogs were unclean creatures by the law; the price of one might not be brought into the house of the Lord, for a vow, 

Deuteronomy 23:18 yea, these creatures were not admitted into several temples of the Heathens . Things profane and unclean, as flesh torn by beasts, were ordered to be given to them.

Exodus 22:31 but nothing that was holy was to be given them, as holy flesh, or the holy oblations, or anything that was consecrated to holy uses; to which is the allusion here. It is a common maxim  with the Jews,, "that they do not redeem holy things, to give to the dogs to eat".''

Here the phrase is used in a METAPHORICAL sense; 

and is generally understood of NOT DELIVERING or COMMUNICATING the HOLY WORD of GOD, & the TRUTHS of the GOSPEL,  holy word of God, and the truths of the Gospel, comparable to pearls, or the ordinances of it, TO PERSONS notoriously vile and sinful: to men, WHO BEING VIOLENT & FURIOUS PERSECUTORS, and impudent blasphemers, are compared to "dogs"; or to such, who are scandalously vile, impure in their lives and conversations, and are therefore compared to swine;

neither cast ye your PEARLS before SWINE. But since the subject Christ is upon is reproof, it seems rather to be the design of these expressions, that MEN should be CAUTIOUS, & PRUDENT, in REBUKING and ADMONISHING SUCH PERSONS for THEIR SINS, in whom there is no appearance or hope of success; yea, where there is danger of sustaining loss;

LEST they TRAMPLE them UNDER their FEET, and TURN AGAIN & REND YOU:  that is, despise the admonitions and reproofs given, and hurt the persons who give them, either by words or deeds;  see Proverbs 9:7. 

source:

http://biblehub.com/commentaries/gill/matthew/7.htm