because you have been built on the foundation of the apostles and prophets, with Christ Jesus himself as the cornerstone.
[Eph 2:20 NET]
The meaning of this expression, which frequently occurs in the New Testament, is, we think, often misunderstood. It is taken in the first instance from the declaration concerning our Lord in the Psalms, which he in the gospels (Mark 12:10; Luke 20:17) specially claimed and appropriated to himself—“The stone which the builders refused has become the head-stone of the corner” (Psalm 118:22). “The head of the corner,” or “the chief corner stone,” the meaning of both expressions being one and the same, signifies not the stone which stands at the top of the building, uniting the corners of the two walls just under the roof, but the broad foundation stone, which is firmly fixed at the very bottom; and it is called the “corner stone” or the “head” or “chief of the corner,” because being laid as a huge and broad stone for a foundation of the whole building, each wall meets upon it at the corners, it equally supporting and upholding them all.
The two walls which thus meet together represent Jew and Gentile; but each of these walls equally rests upon the broad foundation stone which is common to both, and not only supports them separately, but unites them together at the corner, where each meets and rests upon it. It is the expression “head” which has caused the misapprehension of the word “corner stone” to which we have alluded; but the word “head” in Hebrew properly signifies the first or chief; and thus as the foundation is not only the chief stone as supporting the whole, but the first which is laid, so our gracious Lord is not only chief in dignity, but was laid first in place, for the Church was chosen in him. In all things he must have the pre-eminence. Thus he is first in dignity, as the Son of the Father in truth and love; first in choice, God choosing the elect in him; first in suffering, for what sorrows were like his sorrows? first in resurrection, for he is “the first-fruits of those who slept;” first in power, for “all power is given unto him in heaven and in earth;” first in glory, for he is gone before to prepare a place for his people; and we may well add, he is first in their hearts and affections, for he that loves father or mother, son or daughter, more than him is not worthy of him.