What is the meaning of Matthew 1:20?

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References

But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Ghost. (Matthew 1:20 KJV)

But when he thought on these things, behold, an angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Spirit. (Matthew 1:20 ASV)

but while he pondered on these things, behold, an angel of the Lord appeared to him in a dream, saying, Joseph, son of David, fear not to take to thee Mary, thy wife, for that which is begotten in her is of the Holy Spirit. (Matthew 1:20 DBY)

And on his thinking of these things, lo, a messenger of the Lord in a dream appeared to him, saying, `Joseph, son of David, thou mayest not fear to receive Mary thy wife, for that which in her was begotten is of the Holy Spirit, (Matthew 1:20 YLT)

But when he thought about these things, behold, an angel of the Lord appeared to him in a dream, saying, “Joseph, son of David, don’t be afraid to take to yourself Mary, your wife, for that which is conceived in her is of the Holy Spirit. (Matthew 1:20 WEB)

Interlinear

Mt 1:20 But /de/ while he /autos/ thought on /enthumeomai/ these things, /tauta/ behold, /idou/ the angel /aggelos/ of the Lord /kurios/ appeared /phaino/ unto him /autos/ in /kata/ a dream, /onar/ saying, /lego/ Joseph, /Ioseph/ thou son /huios/ of David, /Dabid/ fear /phobeo/ not /me/ to take /paralambano/ unto thee /sou/ Mary /Maria/ thy wife: /gune/ for /gar/ that which is conceived /gennao/ in /en/ her /autos/ is /esti/ of /ek/ the Holy /hagios/ Ghost. /pneuma/ {conceived: Gr. begotten}

Albert Barnes’ Commentary

Verse 20.  He thought on these things. He did not act hastily. He did not take the course which the law would have permitted him to do, if he had been hasty, violent, or unjust. It was a case deeply affecting his happiness, his character, and the reputation and character of his chosen companion. God will guide the thoughtful and the anxious. And when we have looked patiently at a perplexed subject, and know not what to do, then God, as in the case of Joseph,  will interpose to lead us, and direct our way, Ps 25:9.

The angel of the Lord. The word angel literally means a messenger. It is applied chiefly in the Scriptures to those invisible holy beings who have not fallen into sin; who live in heaven, (1Ti 5:21, comp. Jude 1:6) and who are sent forth to minister to those who shall be heirs of salvation, Heb 1:13; 13:25; Da 9:21. The word is sometimes applied to men, as messengers, (Lu 7:24; 9:52; Jas 2:25) to the winds, (Ps 104:4); to the pestilence, (Ps 78:49); or to whatever is appointed to make known, or to execute the will of God. It is commonly applied, however, to the unfallen, happy spirits that are in heaven, whose only dignity and pleasure it is to do the will of God. Various ways were employed by them in making known the will of God, by dreams, visions, assuming a human appearance, etc.  

In a dream. This was a common way of making known the will of God to the ancient prophets and people of God, Ge 20:3; 31:10,11; 37:5; Ge 41:1; 1Ki 3:5; Da 7:1; Job 4:13-16.  In what way it was ascertained that these dreams were from God, cannot now be told. It is sufficient for us to know that in this way many of the prophecies were communicated; and to remark, that now there is no evidence that we are to put reliance on our dreams. Dreams are wild, irregular movements of the mind, when it is unshackled by reason, and it is mere superstition to suppose that God now makes known his will in this way. See Barnes for Isa 37:1.

Saying, Joseph. We are known to angels, and they address us by name (Ac 10:3; 27:24). Much more does the Lord know our names (Joh 10:3; Lu 19:5).  —-The Fourfold Gospel

Thou son of David. Descendant of David. See Mt 1:1. The angel put him in mind of his relation to David, perhaps, to prepare him for the intelligence that Mary was to be the mother of the Messiah–the promised heir of David.

Fear not. Do not hesitate, or have fears about her virtue and purity. Do not fear that she will be unworthy of you, or will disgrace you.

{w} “in a dream” Mt 1:16

{1} “conceived” or, “begotten”