"'If you had known Me, you would have known My Father also; and from now on you know Him and have seen Him.' Philip said to Him, 'Lord, show us the Father, and it is sufficient for us.' Jesus said to him, 'Have I been with you so long, and yet you have not known Me, Philip? He who has seen Me has seen the Father; so how can you say, "Show us the Father"? Do you not believe that I am in the Father, and the Father is in Me? The words that I speak to you I do not speak on My own authority; but the Father who dwells in Me does the works. Believe Me that I am in the Father and the Father in Me, or else believe Me for the sake of the works themselves'" (John 14:7-11).
If a person really knows Jesus, then he also knows the Father. This is the case because the Father and Son, although distinct, share a perfect unity of nature and character. If a disciple truly knew Jesus by observation and experience, then he would know God the Father through the process of reflection. Thomas' question back in John 14:5 is clear evidence of the fact that he and the other disciples hadn't really understood or "known" Jesus in spite of their years of dedication to Him. However, as they would soon finally start to comprehend the spiritual nature of Jesus' work and His kingdom, their understanding of the Father would correspondingly increase.
"Lord, show us the Father, and it is sufficient for us" - As Thomas had asked for a physical way of approach to God, now Philip asks for a physical revelation of the Father. Philip was asking for some sort of divine appearance of the Father (i.e., a theophany), like that which Moses had experienced on Mount Sinai (cf. Exo. 33:18ff). He felt that they would be content if they could just see the Father with their physical eyes one time. However, it is doubtful that such, had it been possible (cf. John 1:18), would have been of any benefit to the apostles since experiencing a physical glimpse of deity would reveal little or nothing about the character and attributes of God. All of the physical demonstrations at Mount Sinai did not even prevent the Israelites from making and worshipping a golden calf (cf. Exo. 19,20,32)!
Jesus' reply is a tender rebuke - "Have I been with you so long, and yet you have not known Me, Philip?" To know Jesus is to see the spiritual qualities which define Him (i.e., righteousness, truth, love, holiness, etc.). These men certainly knew these attributes of Jesus, but they are slow to comprehend the implications of knowing such about the Son of God.
"He who has seen Me has seen the Father" - The apostles had been blessed with several years to "see" Jesus, and consequently, the Father (since He has the same divine attributes as the Son). Some have mistakenly concluded from this context that Jesus and the Father are identical (i.e., Jesus is both the Father and the Son). Such cannot be true for a variety of reasons (e.g., John 14:6,16,23,24,28; etc.).
Jesus had spoken previously of His unity with the Father (cf. John 10:30) and that He acted and taught based on the Father's will, not His own (cf. 8:28,29). The Father and the Son are so united in their character, intent, and work that the action of one is the action of the other.
"Believe Me that I am in the Father and the Father in Me, or else believe Me for the sake of the works themselves" - By saying that He and the Father are "in" each other, Jesus is simply referring to their close association. Jesus asks Philip (and the others) to take His word for this, since a physical demonstration could not be made of this spiritual unity. If one was reluctant to simply take Jesus at His word on this matter, then the divine works that Jesus had performed should have been sufficient testimony to the presence of a divine spirit and power within Jesus.