To deny the Son is not to have the Father; to confess the Son is to have the Father. John uses this phrase to indicate that the Son and the Father go hand in hand in the Christian faith.
Christianity is a relationship not a religion. Religion is simply mans useless attempt to imitate a theology which empowers mankind to advance toward God (the Father) from under his (mans) own energy and rules.
The Bible rejects all of mankind’s efforts.
He who does not honor the Son does not honor the Father who sent Him. NASB
Even though man has invented many religions, and many of these religions presume to have a calling toward God (the Father), they reject Christ, the Son of God. Therefore, they reject the Father in so doing.
The Father set out a plan for humanity and all of history. Part of that plan is the utilization of the Second person of the Trinity to execute that plan. Likewise the Third person of the Trinity is the prime communicator of that plan. All three go together. No one of the persons of the Trinity can be separated from the other two even for the convenience of mans alternative beliefs.
This is why all religions are evil. They are not of God, they stand in total rejection of all that God is. There is no such thing as an honorable religion regardless of its pretense of teaching peace, or humility, or the goodness of mankind.
Without God and doctrine, these things cannot exist. Man cannot bring them into existence. If a thing is not the truth, then it is a lie.
Likewise, John is speaking to believers in this book. So now the pretense is switched from an alternative religion, to an alternative spirituality.
There is only one path to the spiritual life, and that path, once you are saved, comes in the form of obedience to Gods Word, and that obedience begins with fellowship with the Father (resulting from ones confession of ones sins – to the Father).
We have the manifestation of Jesus Christ in the scriptures. Once we have come to see Christ, then we have seen the Father as well. They are one and the same in essence. To reject one, means rejection of both.
Since Jesus Christ is the living Word, then any alternative scriptures or compromised beliefs, or selectivity in what you will or will not believe from within the scriptures is a rejection of the Word and of Christ, and the Father.
You cannot play games with God. You cannot redefine His will, His word, His plan, His essence. You cannot do anything simply for the sake of your own convenience.