There does not appear to be a clear explanation as to how the two interwoven equilateral triangles came to be known as the "Star of David," or the "Shield of David." I will leave that discussion to the "experts." I would like to share my theory on what the symbol represents.
To begin with, both God and mankind are described as triune beings in the Bible. That there is one God, is clear: "Hear, O Israel: The LORD our God is one LORD" (Deut. 6:4). Yet, we find God referred to as: 1) The Father of Jesus Christ (2 Cor. 11:31); 2) The Son of God (Cp. Ps. 45:6 w/ Heb. 1:8; Ps. 110:1 w/ Heb. 1:13); and 3) The Holy Spirit (Acts 5:3-4). Man is also described as consisting of a body, a soul, and a spirit. 1 Thessalonians 5:23 says, "And the very God of peace sanctify you wholly; and I pray God your whole spirit and soul and body be preserved blameless unto the coming of our Lord Jesus Christ."
If we were to picture the equilateral triangle in the "Star of David" having two corners of the triangle at its top, and one corner at the bottom, it would serve as a picture of the Godhead as we understand it today. The Father is seated on His throne in heaven, and the Son is seated at His right hand (Eph. 1:20). The single, bottom point, is a picture of the Holy Spirit at work on the earth building the Body of Christ. He has been sent by both the Father and the Son (Jn. 14:26; 15:26; Acts 1:4-5).
The equilateral triangle representing Man has a single point pointed upward, and two points at the base. The bottom two points represent that part of Man that is described to be "earthy" (1 Cor. 15:47-49); that is, the body and the soul. The top point represents the spirit of the born again believer, said to be seated with Christ in the heavenly places (Eph. 2:6).
Together, these two equilateral triangles represent the
God-Man, Jesus Christ, the Son of David!