JD, Psalm 118 does not need to mention the apostles for Christ to be identified as the cornerstone before His resurrection. The cornerstone is the Messiah Himself, not the apostolic group. Psalm 118:22 says, “The stone which the builders refused is become the head stone of the corner.” Jesus applies this to Himself in His earthly ministry in Matthew 21:42, before His death, and Peter applies it again in Acts 4:11 after His resurrection. The text shows the Messiah as the cornerstone in prophecy, in His ministry, and in His exaltation.
Your reading of Psalm 118 as a national prophecy of Israel in the millennial day is your interpretive framework, but Jesus Himself applies the psalm directly to His own person, not to Israel collectively. Matthew 21:42 is explicit: “Jesus saith unto them, Did ye never read in the scriptures, The stone which the builders rejected…?” He identifies Himself as the rejected stone during His earthly ministry. That is the point I made.
Regarding the beginning of Christ’s building work, Luke 6:13 records the moment He constituted the apostles: “He called unto him his disciples: and of them he chose twelve, whom also he named apostles.” Ephesians 2:20 identifies these men as the foundation of the assembly Christ promised to build. The forming of the foundation is the beginning of the building. Pentecost is the empowering of the assembly, not the beginning of its formation.
Christ is the cornerstone in prophecy (Psalm 118), in His ministry (Matthew 21), and in His exaltation (Acts 4). He began building when He laid the foundation stones in Luke 6. That is the Scriptural sequence I am presenting.
Psalm 118 as well as all the psalms is a prophecy.
Lk 6:12 And it came to pass in those days, that he went out into a mountain to pray, and continued all night in prayer to God.
13 And when it was day, he called unto him his disciples: and of them he chose twelve, whom also he named apostles;
14 Simon, (whom he also named Peter) and Andrew his brother, James and John, Philip and Bartholomew,
15 Matthew and Thomas, James the son of Alphaeus, and Simon called Zelotes,
16 And Judas the brother of James, and Judas Iscariot, which also was the traitor.
17 And he came down with them, and stood in the plain, and the company of his disciples, and a great multitude of people out of all Judaea and Jerusalem, and from the sea coast of Tyre and Sidon, which came to hear him, and to be healed of their diseases;
18 And they that were vexed with unclean spirits: and they were healed.
19 And the whole multitude sought to touch him: for there went virtue out of him, and healed them all.
46 And why call ye me, Lord, Lord, and do not the things which I say?
47 Whosoever cometh to me, and heareth my sayings, and doeth them, I will shew you to whom he is like:
48 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. (see Eph 2:19-22)
49 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.
One should read this whole chapter because the instructions he is giving these men he has chosen reflects the spirit and attitude that he has ministered with. This is very near the end of his earthly ministry and he will be leaving shortly to go to the Father to receive for himself a kingdom. But he is leaving stewards to care for his possessions while he is away. He is giving them the very kingdom powers that he has ministered with so they can do all the miracles that he has done. Jesus according to several passages in the scriptures ministered from his conception with the power of the Holy Ghost. Late on resurrection day Jesus met with these men and breathed the Holy Spirit into 10 of them (John 20) and the authority to bind and loose even to the raising of the dead. The Spirit would be in full control of their work as he had been the power of Jesus during his incarnation.
There is much I can say about this but I want to bring your attention with the remainder of this post to a parable of Jesus on the day he came to Jerusalem a week before his crucifixion. This is important. Notice the ten's in this parable which is also a prophecy.
10 For the Son of man is come to seek and to save that which was lost.
11 And as they heard these things, he added and spake a parable, because he was nigh to Jerusalem, and because they (his disciples which included the apostles) thought that the kingdom of God should immediately appear.
12 He said therefore, A certain nobleman (Jesus) went into a far country to receive for himself a kingdom, and to return.
13 And he called his ten servants, and delivered them ten pounds, and said unto them, Occupy till I come.
(See Acts 1-7 here)
14 But his citizens Judah) hated him, and sent a message after him, saying, We will not have this man to reign over us.
Lk 19:15 And it came to pass, that when he was returned, having received the kingdom, then he commanded these servants to be called unto him, to whom he had given the money, that he might know how much every man had gained by trading.
16 Then came the first, saying, Lord, thy pound hath gained ten pounds.
17 And he said unto him, Well, thou good servant: because thou hast been faithful in a very little, have thou authority over ten cities.
18 And the second came, saying, Lord, thy pound hath gained five pounds.
19 And he said likewise to him, Be thou also over five cities.
20 And another came, saying, Lord, behold, here is thy pound, which I have kept laid up in a napkin:
21 For I feared thee, because thou art an austere man: thou takest up that thou layedst not down, and reapest that thou didst not sow.
22 And he saith unto him, Out of thine own mouth will I judge thee, thou wicked servant. Thou knewest that I was an austere man, taking up that I laid not down, and reaping that I did not sow:
22 And he saith unto him, Out of thine own mouth will I judge thee, [thou] wicked servant. Thou knewest that I was an austere man, taking up that I laid not down, and reaping that I did not sow:
23 Wherefore then gavest not thou my money into the bank, that at my coming I might have required mine own with usury?
24 And he said unto them that stood by, Take from him the pound, and give [it] to him that hath ten pounds.
25 (And they said unto him, Lord, he hath ten pounds)
26 For I say unto you, That unto every one which hath shall be given; and from him that hath not, even that he hath shall be taken away from him.
27 But those mine enemies, which would not that I should reign over them, bring hither, and slay them before me.
Jesus here is giving what he knows will be their response to these apostles, not what he sovereignly predetermined for them. Every man will give an account for his own actions and those actions will be the tell tale evidence for others to see. His divisions of the ages are set in eternity and will take place on time .In this age justification is believing that Jesus Christ is the virgin born son of God and his death, burial, and resurrection saves us from the penalty of our sins, which is the second death in the lake of fire and gives us hope of our own resurrection and glorification. The eye witness testimony of the apostles and the 70 elders that Jesus chose in Luke 10 and what they wrote and God preserved for 2000 years in his Bible is the foundation of our faith. In this way they are the foundation of the church and these men could not have written a word about the gospel of salvation before the events of the cross because not a one them believed when they heard that Jesus had risen from the dead. Before the cross they believed what at their time brought justification and that is clearly presented in John 20:30-31 as being that "Jesus is the Christ the son of God and that believing they will have life through his name."
I am on solid ground here and the apostles link the gentile church to the kingdom of God.