Is God's Law-Word Valid For Us Today?

You have heard it said:

A very important information is that we, believers from gentiles were never given the Old testament Law. The OT Law was given only to the Jews.
We are not under the Law. So even in the polygamy was prevalent in the times of the OT it cannot be any basis for the church to stand on and look to.
When Jesus came He changed the Law. He taught the Jews in Mat 5:21 "Ye have heard that it was said by them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment:"
and in the verse 22 Jesus clearly says: "But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire. "
Jesus introduced a new law (the law of love), a new covenant.

First, I would note that "Jews" or more specifically, members of the Tribe of Judah, are only one tribe out of twelve, so to say that the Law-Word of God was given only to the Jews is factually incorrect. God's Law-Word was given to all 12 of the Tribes of the Nation of Israel.

Secondly, polygyny is a Biblical form of marriage.  Do you mean to imply that marriage is now sin because it wasn't commanded in the Old Testament or because it is in teh New Testament?

Matthew 5:21 is not an example of Jesus changing HIS OWN Law-Word.  On the contrary, he was restating the same conditions of the covenant, but adding the additional requirement of it coming from the heart, and not by the letter alone.

Is the Law-Word of God null and void to us today?  God forbid! 

The New Testament teaches us that--unless exceptions are revealed elsewhere--every Old Testament commandment is binding, even as the standard of justice for all magistrates (Rom. 13:1-4), including every recompense stipulated for civil offenses in the law of Moses (Heb 2:2). From the New Testament alone we learn that we must take as our operating presumption that any Old Testament penal requirement is binding today on all civil magistrates. The presumption can surely be modified by definite, revealed teaching in the Scripture, but in the absence of such qualifications or changes, any Old Testament penal sanction we have in mind would be morally obligatory for civil rulers.

Greg Bahnsen, No Other Standard (Tyler, TX: Institute for Christian Economics, 1991), p. 68.

The principle of interpretation which is supposed to govern Christian orthodoxy is that Christ came to establish, confirm, and declare the Old Testament law (Matt. 5:17-18). Only if we find an explicit abandonment of an Old Testament law in the New Testament, because of the historic fulfillment of the Old Testament shadow, can we legitimately abandon a detail of the Mosaic law. Many have done a reformed critique and those that do seem to go to far as to even deny the importance of keeping the Ten Commandments. The question should really be how much should our government apply of the moral law today and are we commanded to partially or completely reform our laws?

The proper exegetical principle is this: Mosaic law is still to be enforced, by the church or the State or both, unless there is a specific injunction to the contrary in the New Testament.

Gary North, The Sinai Strategy: Economics and the Ten Commandments (Tyler, TX: Institute for Christian Economics, 1986), pp. 242, 255.

One of the most important and most misunderstood of all Biblical declarations concerning the law is our Lord's declaration in the Sermon on the Mount: Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil. For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled (Matt. 5:17,18).

Two different words are used for the idea of fulfillment. The word translated as "fulfil" in verse 17 is plerosai, related pleroma; it means to make full, to the top, to fill, diffuse, to cause to abound, to pervade. Christians are said to be plervusthai, filled with the power of the Holy Spirit (Col. 2:10; Eph. 3:19). Christ "fills" the universe with His power and activity (Eph. 4:10, pleroun). The word means to fill and to keep full, i.e., to put into force as a continuous thing. Thus, our Lord declared that He had come to put the law into force and to keep it in force. . . Christ as the Messiah or King, because He has come, declared afresh the validity of the law and His purpose to put it into force.

R.J. Rushdoony, The Institutes of Biblical Law (Nutley, NJ: Craig Press, 1973), p. 698.

Now, Let's take a look at First and Second John.

(1 John 2:3 KJV)  And hereby we do know that we know him, if we keep his commandments.

(1 John 2:4 KJV)  He that saith, I know him, and keepeth not his commandments, is a liar, and the truth is not in him.

And

(1 John 2:7 KJV)  Brethren, I write no new commandment unto you, but an old commandment which ye had from the beginning. The old commandment is the word which ye have heard from the beginning.
(1 John 2:8 KJV)  Again, a new commandment I write unto you, which thing is true in him and in you: because the darkness is past, and the true light now shineth.

And

(1 John 3:22 KJV)  And whatsoever we ask, we receive of him, because we keep his commandments, and do those things that are pleasing in his sight.
(1 John 3:23 KJV)  And this is his commandment, That we should believe on the name of his Son Jesus Christ, and love one another, as he gave us commandment.
(1 John 3:24 KJV)  And he that keepeth his commandments dwelleth in him, and he in him. And hereby we know that he abideth in us, by the Spirit which he hath given us.

And

 (1 John 5:2 KJV)  By this we know that we love the children of God, when we love God, and keep his commandments.
(1 John 5:3 KJV)  For this is the love of God, that we keep his commandments: and his commandments are not grievous. 

And

 
(2 John 1:4 KJV)  I rejoiced greatly that I found of thy children walking in truth, as we have received a commandment from the Father.
(2 John 1:5 KJV)  And now I beseech thee, lady, not as though I wrote a new commandment unto thee, but that which we had from the beginning, that we love one another.
(2 John 1:6 KJV)  And this is love, that we walk after his commandments. This is the commandment, That, as ye have heard from the beginning, ye should walk in it.