Understanding the Foundation of the Church

There are two verses of Scripture in Luke's Gospel which help our understanding regarding who or what is the foundational rock upon which the Church is built. These verses are found in Luke 6:48,49: He is like a man which built an house, and digged deep, and laid the foundation on a rock: and when the flood arose, the stream beat vehemently upon that house, and could not shake it: for it was founded upon a rock. But he that heareth, and doeth not, is like a man that without a foundation built an house upon the earth; against which the stream did beat vehemently, and immediately it fell; and the ruin of that house was great.

The term 'foundation' is used in two ways here:

Firstly, in v49, the term is used to refer to the immovable rock that lies beneath the surface of the earth. This rock is here used in contrast to the movable 'earth', upon which a man seeks to build a house. The earth is akin to the clay, or sand, as Matthew mentions in his similar account, cf. Matthew 7:26: And every one that heareth these sayings of mine, and doeth them not, shall be likened unto a foolish man, which built his house upon the sand. The earth has no strength or ability to hold up the structure built upon it. There is the rock, and there is the sand, upon which the whole structure can be built. Only the house built upon the rock will stand.

Christ Jesus is this rock upon which the whole edifice of the Church is built. He is 'Petra', the rock.

Secondly, in v48 the term is used to refer to the foundation that is laid down by the builder. Even today we speak about builders 'laying a foundation'. This foundation needs to be laid upon solid ground or it too will give way eventually. The builder, in this illustration in Luke's Gospel, lays his foundation on a rock. The house he subsequently builds upon the rock will stand.

In ancient times builders used large stones for this 'foundation' that they would lay down. Many ancient buildings have massive 'foundation stones'. This was part of the wonder of the Temple, to which the disciples drew the Saviour's attention, cf. Matthew 28:1; Mark 13:1; Luke 21:5.  Some of these stones were estimated at around 20 metres in length. The 'cornerstone' within this element of the 'foundation' was vitally important, as structurally these 'cornerstones' carried so much of the weight of the building.

From Scripture we learn that Jesus Christ is again part of this foundation. He is the chief cornerstone, cf. 1 Peter 2:6: Wherefore also it is contained in the scripture, Behold, I lay in Sion a chief corner stone, elect, precious: and he that believeth on him shall not be confounded. A quotation taken from Isaiah 28:16.

The Apostles are also part of this secondary 'foundation'. They are the other foundational stones, cf. Ephesians 2:20: And are built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief corner stone.

Therefore, Luke 6:48,49 helps us to understand who the rock must be in Matthew 16:18. The rock, upon which everything is built, must be Jesus Christ, as He alone is the foundation no man has laid but God, cf. 1 Corinthians 3:11: For other foundation can no man lay than that is laid, which is Jesus Christ. The secondary foundation laid down is Christ, the chief corner stone and the Apostles. 

I would understand the two ways in which Christ is the foundation of the Church to refer to His person and then to His work.