This verse could be interpreted in one of two ways:
1) a promise
If you really knew Me, you will know My Father as well.
2) a rebuke
If you really knew Me, you would know My Father as well.
It would seem that the Lord is rebuking them for a failure to understand His person and mission. Compare http://ref.ly/Jn8.19. The following dialogue indicates a failure on the disciples’ part. From now on, you do know Him is a promise, which looks beyond the Cross and the Resurrection.
http://ref.ly/Jn20.28. My Lord and my God.
Philip expressed the desire of all humanity, to see God. . This might lead to idolatry in the life of the believer. Philip probably longed for a theophany. http://ref.ly/Ex24.9f and http://ref.ly/Is6.1. He may have sought some visible display of God’s glory. “Anyone who has seen Me has seen the Father” http://ref.ly/Jn12.45, is one of the most staggering claims He ever made. The Father is in Jesus, and Jesus perfectly reveals Him John 1:18. There was no need for a theophany By seeing Jesus, they were seeing the Father!
The proof of the union of Jesus and His Father is threefold. Jesus’ disciples should believe Jesus for the following reasons:
a) Because of His character (I am in the Father (v. 20…and the Father is in Me
b) Because His words are the Father’s (The words I say to you are not just My own (cf. 7:16; 12:49-50; 14:24)
c) Because the miracles reveal God’s working through Him (the Father, living in Me…is doing His work…believe on the evidence of the miracles themselves; John 5:36. One of the key elements in John’s Gospel is the stress on the signs as gracious pointers to faith. Look at the following verses: http://ref.ly/Jn5.36, http://ref.ly/Jn10.25 and http://ref.ly/Jn11.47.