“know her.” Matthew 1:24 says Joseph took Mary home as his wife, but Matthew 1:25 makes it clear that he did not have sexual intercourse with her until after Jesus was born. Mary’s being pregnant would not have stopped her from having sexual intercourse, so why would they have waited until after Jesus was born? Before we answer that question, we need to be sure we correctly understand what this verse is saying.
In both Hebrew and Greek, the word “know” was a common idiom for having sexual intercourse (cp. Gen. 4:1), even including rape (Gen. 19:5; Judg. 19:25). Other idioms for sexual intercourse include, “go into” (2 Sam. 3:7), and “go near; approach” (Exod. 19:15), “be with” someone (2 Sam. 13:20), “uncover the nakedness” (Lev. 18:12); and sometimes “see the nakedness” (Lev. 20:17).
The Hebrew word “know” that is used idiomatically for sexual intercourse is the common word yada (#03045 ידע), which is used well over 800 times in the Old Testament. Its first use in the Bible for sexual intercourse is Genesis 4:1, and it is used many other times that way (cp. Gen. 4:1, 17, 25; 19:8; 24:16; Judges 11:39; 19:25; 1 Sam. 1:19; 1 Kings 1:4). In the Greek New Testament, the word “know” is the common word ginōskō (#1097 γινώσκω), which occurs more than 200 times and is used of “knowing” someone sexually in Matthew 1:25 and Luke 1:34. The association between sex and “knowing” was most likely made because modesty and sexual privacy are normal parts of our humanity, so if we become sexually intimate with someone, we “know” them in a unique and personal way. Interestingly, through the ages, spies have used the special connection and intimacy that comes with “knowing” someone sexually to get to know other things about them, including top-secret information, because knowing a person sexually often leads to intimate knowledge in other areas as well.
From a lexical viewpoint, “knowing” someone sexually, which involves intimate and experiential knowledge, is quite close to the ordinary semantic range of the word “know,” which includes thorough or experiential knowledge as well as just intellectual knowledge. For example, when the Bible says that Jesus “knew” no sin (2 Cor. 5:21 KJV), it is not that he did not have intellectual knowledge of sin, but rather that he had no experiential knowledge of sin. Similarly, when Romans 3:17 says the wicked have not “known” the way of peace, it is not saying that the wicked do not know what peace is, but they have not experienced it. It is possible that “know” as an idiom for sexual intercourse came into the Greek language after the Greeks conquered Israel and Egypt, because “know” is used for sexual intercourse from the time of Alexander the Great down. In any case, “know” shows up as an idiom for sexual intercourse in the writings of Greek authors such as Menander of Athens, Hiraclides, Plutarch, and the apocryphal book of Judith (Thayer; BDAG; EDNT).
It has been suggested by at least one theologian that “know” refers to sexual intercourse that results in conception, but that is not accurate, something that can be seen by studying the Hebrew and Greek writings and lexicons. In the OT, verses such as Genesis 19:8 and Judges 19:25 clearly make a separation between sexual “knowing” and conception. Similarly, in the New Testament in Luke 1:34, when the angel tells Mary she will be pregnant, she replies, “How will this be, since I am not knowing a man” (literal translation). In Luke 1:34, the verb “know” is present tense, active voice, which indicates action that is currently going on in the present. Thus, when Mary told the angel she was not “knowing” a man, she was saying she was not actively having sex at the time, in her case, because she was not married. It is clear from Mary’s statement to the angel that she did not think “know” included conception. Furthermore, verses such as Genesis 4:1, 17 and 4:25, show that the “knowing” and the conception were two separate events because the verses say that the husbands “knew” their wives and also say they “conceived.” If “know” included conception, then adding the phrase “and she conceived” would have been inappropriate.
The question remains as to why Joseph did not have sexual intercourse with Mary until after Jesus was born, and the answer is both simple and profound. He wanted there to be no doubt in anyone’s mind that Jesus was not his child. Today we track pregnancies with a precise knowledge of when the baby will be born, and women who have only been pregnant for a couple months speak of their “due date” months away. In our world of precise due dates and DNA paternity testing, Joseph could prove he was not Jesus’ father. However, things were much less clear in the ancient world. For example, Sarah Pomeroy notes that the exact period of gestation was not known in ancient times, and she writes, “Some Romans believed that children could be born seven to ten months after conception, but that eight month babies were not possible” (Goddesses, Whores, Wives and Slaves; Shocken Books, New York. 1975, p. 168). There was just no way Joseph could conclusively prove that he was not Jesus’ father if he had sexual intercourse with Mary before Jesus was born. In fact, the uncertainty about paternity was one reason kings did not let women leave their harems, even if they were no longer interested in them, and why David wanted his first wife Michal to be in his harem even though he had other wives and she had married another man during the years David was running from Saul (2 Sam. 3:13-15). If a woman who had been with a king had a baby, it could be set forth as an heir of the king without there being a way to disprove the claim. And even if the paternity was unlikely, no king wanted to take the risk of having a possible heir and rival out causing trouble.
Joseph was a godly and honorable man, and did not want to cast any doubt on the fact that God was the father of the Lord Jesus Christ, so he and Mary restrained their passions and acted in the best interests of God and Jesus, waiting until after Jesus was born to have sexual intercourse. Matthew 1:25 makes it clear that the birth of our Lord and Savior Jesus Christ was indeed a virgin birth, one of the great miracles in history, and because Joseph and Mary waited to have sex, we can be sure that God is the Father of the Lord Jesus Christ.