According to Philippians 2:9... Jesus is the Name above all names. This would compliment Isaiah 9:6. And... it does so quite well.
This makes it easy to know with biblical assurance that Jesus is Avi-'ad... as Isaiah says.
And... according to 1 John 4:2 -(
side track 1 John 4:2 can be misinterpreted... easily... by people that don't believe that Jesus is God...
it is literally saying that to deny that "God"... expressed with the Name above all names...
came in "flesh". Once the flesh thing is said... Romans 8:3 must be noted... "likeness" is not "sinful flesh"... lest we accidentally suggest God had ever sinned...
is a very bad idea. end of sidetrack)- and 2 John 1:7 ... it's a pretty bad idea to suggest that Avi-'ad didn't walk amongst us in flesh.
This makes a solid case to say... in answer to your question...
Is the Father the only true God?
NO! To say there is "Only" the "Father" would be to deny the "Mystery" of 1 Timothy 3:16 and Colossians 2:9 .
In other words.. we would unknowingly be discarding our hope and salvation, by denying that the Father and the Son are indeed ONE... but at the same time... to deny that the Son is the presence of Avi-''ad married to humanity.
Literally...
Son of God (1 John 4:15)
Son of Man (Matthew 9:6)
Any tampering with this truth is "divorcing" ourselves from the "BrideGroom". (Ephesians 5:30)
Also... to deny that the Father and Son experienced a deep relationship that was genuine... is again a trashing of God's Revelation of Love towards us. (Galatians 4:6 + Romans 8:9 + Philippians 1:19)
I would be glad to express the Holy Spirit as well... but I wanted to focus on your question.... though... Galatians 4:6 would be a good Segway.