The continuing dialog of Jesus in John chapters 13,14, and 15 was spoken to comfort his disciples through what was to be their toughest time, his arrest and crucifixion. One of the themes of comfort was to emphasize the unity between himself and the Father. Yet the disciples knowledge of the deep things of
God, at that time, was imperfect. In order for us to appreciate the words of the Lord found in John 14:9, we need to back up a several verses and capture the hunger for clarity in the questions of Thomas/Phillip and the masterful sharpness of our Lord's answer.
John 14:1 Let not your heart be troubled: ye believe in God, believe
also in me.
It goes without saying how crucial having faith in Christ is in our relationship with the Father. You cannot have true faith without Christ. But here Christ is not simply requesting his disciples belief, they'd confirmed that several times. He was saying that in the coming troubles, and in spite of their feelings of doubt, their comfort should rest in Him, and His words, as much as in the Father in heaven.
Jesus had told Peter that he would deny him three times some time before (ch 13). And later Philip demands some manifestation of the Father to ease his worried mind. So these were times of intense worry. But back to vs 2-3.
John 14:2 In my Father's house are many mansions; if it were not so,
I would have told you; for I go to prepare a place for you. John 14:3
And if I go and prepare a place for you, I come again, and will
receive you unto myself; that where I am, there ye may be also.
Here Jesus takes their minds from the earthly things to heavenly things. Again, notice the unity of purpose in his words. He assures them the things to be done in heaven will be done by none other than by Him. And those things that were to be done are purposed in His unity with the Father.
John 14:4 And whither I go, ye know the way.
Here Jesus alludes to something he'd told his disciples several times before, his path to the cross and subsequent ascension to heaven. Although they'd heard Him speak of the cross before they either did not understand it, want to understand it, or downright rejected it as Peter did in Matt 16:21-23. Peter was rebuked in Matt 16 because he denied the way salvation was to come, the cross. Jesus told them emphatically they knew the way because they'd had similar conversations before on the topic. It mattered little that they did not want to hear it.
John 14:5 Thomas saith unto him, Lord, we know not whither thou
goest; how know we the way? John 14:6 Jesus saith unto him, I am the
way, and the truth, and the life: no one cometh unto the Father, but
Jesus words are clear to us, but of course were dim to his disciples. The way to the Father can come by no other path than Christ's work of the cross.
As we approach the core of your question, keep in mind several things. God is indeed a Spirit. Yet He has manifested Himself to prophets throughout history. In Daniel 7:9-10, 13-14 He physically manifested His glory as the Ancient of Days. In Revelation 4:1-11 and 5:1-14 he appeared to John (notice John saw His right hand Rev 5:1). Yet we can not logically understand the being of God the Father other than existing as a Spirit who reveals Himself to mankind as only He chooses to. Paul says this of God:
1Tim 6:16 who only hath immortality, dwelling in light
unapproachable; whom no man hath seen, nor can see: to whom be
honour and power eternal. Amen. RV
But there is indeed a way we do see the Father:
Joh 1:18 No one has ever seen God. The unique God, who is close to
the Father's side, has revealed him. ISV
St. John says that Jesus reveals the Father to mankind. Not dimly, but perfectly. Consider this verse.
Heb 1:3 who being the brightness of his glory and the express image
of his substance and upholding all things by the word of his power,
when he had by himself purged our sins, sat down on the right hand of
the Majesty on high;
So it can be said, without hesitation, that to see Christ is to see the Father. Jesus will forever be man, and will eternally be our physical manifestation of who God is. In fact, Jesus not only claimed to be one with the Father (John 10:30), He expresses his unity with the Holy Spirit this way:
Joh 14:17 even the Spirit of truth, whom the world cannot receive
because it does not see him, or know him; but ye know him, for he
dwells with you and shall be in you. Joh 14:18 I will not leave you
orphans; I will come to you.
After Jesus speaks of His departure, He claims the Holy Spirit will come to them, and by way of relationship its the same as Jesus coming to them.
Now for your answer
John 14:7 If ye had known me, ye should have known my Father also; and from now on ye know him and have seen him.
John 14:8 Philip said unto him, Lord, show us the Father, and it will suffice us.
John 14:9 Jesus said unto him, Have I been such a long time with you, and yet thou hast not known me, Philip? He that has seen me has seen the Father; and how sayest thou then, Show us the Father?
John 14:10 Believest thou not that I am in the Father and the Father in me? The words that I speak unto you I speak not of myself; but the Father that dwells in me, he does the works.
Joh 14:11 Believe me that I am in the Father and the Father in me, or else believe me for the very works' sake.
When Jesus responded to Philip's request to see the Father, it was to point Phillip and the other disciples minds to one deep and indisputable truth: That the unity of one God in three persons is so unified in purpose, being, and glory, that the person and works of Christ imprints the same indelible marks on creation as the Father we pray to in heaven. When Jesus commented that they should have believed in Him for the works he'd done vs 11, they should have been at least mildly grieved. His works were done to convince the hard at heart to acknowledge who He was. To have seen Jesus, as all who witnessed Him on earth, was the same as seeing the Father. No, not literally as Christ's manifestation as man (Servant) does not allow that. But in the truest sense of being and majesty.