The biblical evidence :
Matthew 1:18-25
1. Joseph and Mary were betrothed (1:18, 20, 24), a relationship regarded as the legal equivalent of marriage. In other words, betrothal could be broken only by a formal divorce. This is why Joseph is referred to as her “husband” (v. 19).
2. Although betrothed, the relationship had not yet been consummated sexually (see vv. 18, 25; also Luke 1:34).
3. Mary’s pregnancy is attributed to the Holy Spirit:
a. Verse 20 – “of the Holy Spirit.”
b. Verse 16 – “and to Jacob was born Joseph the husband of Mary, by/of whom [feminine] was born Jesus.” Matthew clearly excludes Joseph.
c. Note that the repeated active verb (“was the father of” or “begot”) gives way to a divine passive in v. 16 (i.e., God is the active agent in the conception and birth of Jesus).
4. Joseph is instructed to take Mary into his house and to name the child (vv. 20-21) thereby establishing for Joseph legal paternity of the child. Hence the community came to believe that Joseph was Jesus’ father (Lk. 2:48; Mt. 13:55).
Luke 1:26-38
1. Mary is explicitly identified as a “virgin” (parthenos, v. 27), a fact she confirms in v. 34.
2. Verse 35 clearly attributes the conception to the work of the Holy Spirit.
3. The terms translated “come upon” and “overshadow” (v. 35) are not euphemisms for sexual relations. They are simply figurative expressions for divine intervention by which God will supercede the natural order of things.
4. For the term “overshadow,” see Ex. 40:35; Psm. 91:4; 140:7; Mt. 17:5; Mk. 9:7; Lk. 9:34 (cf. also Gen. 1:2). The emphasis is on the powerful creative presence of the Spirit in bringing to pass the conception of the man Jesus.
Sam Storms