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How about polygamy in the Old Testament? David was given permission to have many wives and he was deemed to be righteous by the Lord and by Nathan, the prophet at that time. It was only when David took on wives that weren't permitted by the Lord that it was wrong. From the Old Testament:

2 Samuel 12: 7,8:
7 And Nathan said to David, Thou art the man. Thus saith the LORD God of Israel, I anointed thee king over Israel, and I delivered thee out of the hand of Saul;
8 And I gave thee thy master's house, and thy master's wives into thy bosom, and gave thee the house of Israel and of Judah; and if that had been too little, I would moreover have given unto thee such and such things.

And how about Abraham? From Genesis 25:1-6

1 Then again Abraham took a wife, and her name was Keturah.
2 And she bare him Zimran, and Jokshan, and Medan, and Midian, and Ishbak, and Shuah.
3 And Jokshan begat Sheba, and Dedan. And the sons of Dedan were Asshurim, and Letushim, and Leummim.
4 And the sons of Midian; Ephah, and Epher, and Hanoch, and Abida, and Eldaah. All these were the children of Keturah.
5 ¶ And Abraham gave all that he had unto Isaac.
6 But unto the sons of the concubines, which Abraham had, Abraham gave gifts, and sent them away from Isaac his son, while he yet lived, eastward, unto the east country.

Later in the Old Testament, Abraham is called a friend of God, ((2 Chron. 20:7, Isaiah 41:8.) In the New Testament, Christ himself spoke respectfully of Abraham ((Matt. 8:11, 22:31-32; Mark 12:26; Like 16:23-30, 20:37; John 8:39,56). And it was after the angel of the Lord stayed Abraham's hand and promised Abraham that his seed would be more numerous than the grains of sand. From Genesis 22:10-16:

10 And Abraham stretched forth his hand, and took the knife to slay his son.
11 And the angel of the LORD called unto him out of heaven, and said, Abraham, Abraham: and he said, Here am I.
12 And he said, Lay not thine hand upon the lad, neither do thou any thing unto him: for now I know that thou fearest God, seeing thou hast not withheld thy son, thine only son from me.
13 And Abraham lifted up his eyes, and looked, and behold behind him a ram caught in a thicket by his horns: and Abraham went and took the ram, and offered him up for a burnt offering in the stead of his son.
14 And Abraham called the name of that place Jehovah-jireh: as it is said to this day, In the mount of the LORD it shall be seen.
15 ¶ And the angel of the LORD called unto Abraham out of heaven the second time,
16 And said, By myself have I sworn, saith the LORD, for because thou hast done this thing, and hast not withheld thy son, thine only son:
17 That in blessing I will bless thee, and in multiplying I will multiply thy seed as the stars of the heaven, and as the sand which is upon the sea shore; and thy seed shall possess the gate of his enemies;
18 And in thy seed shall all the nations of the earth be blessed; because thou hast obeyed my voice.

Isaac was not Abraham's firstborn child, he had another with Hagar (Ishmael.)