Revelation 2:3-4

3 I know you are enduring patiently and are bearing up for My name's sake, and you have not fainted or become exhausted or grown weary. 

4 But I have this [one charge to make] against you: that you have left (abandoned) the love that you had at first [you have deserted Me, your first love]. 

The Holy Spirit really had Paul write to the Ephesians about our oneness with Jesus in the Book of Ephesians. We can see what our Father had to say to them about being one with Him in: Ephesians 2:1, 2:6, 2:13, 2:15, 3:17, 4:6, 5:8, 5:30, and 5:32. I believe that many in the church of Ephesus were saved, yet the leadership, or the messenger, of the organized body was something else, just like in so many churches today. Jesus cannot be fooled; He knows the beginning and the end. The letter, or what we know as the Book of Ephesians, that Jesus had Paul write, was to true believers who knew what and who Jesus was, and they knew their relationship to Him. But as for the leadership of Ephesus, they had left Jesus, because they were never of Jesus. We know this because Jesus does not let His Bride leave Him. What kind of Shepherd would He be if His true sheep were allowed to wonder off? Can a branch decide to leave the Vine? Can an arm leave the body? Jesus tells us to have any part in the Kingdom of Heaven we have to be as little children, Matthew 18:3. Does a father allow his little children to leave his parenting and care? If our Father would allow His little children to wonder off, then He would be guilty of child abuse! These leaders, who had forgotten their first love, obviously had no root, they appeared to be with Jesus, but the Truth was they were never of Him. 

It is simple, the Truth is always simple, and the Bible is simple. If anyone tries to tell you that it isn’t, don’t listen to them. We have the very Spirit of our Father within us, not just in us, but one with us, and the Holy Spirit teaches us all things, John 14:26. The Bible is not difficult for the Holy Spirit and the simple Truth is that He is the One teaching us. He knows each of us, He knows how we are and what we are, and He knows how to make each one of us learn the Truth that will set us free. The Truth is that we are either of our Father or we are not. If we are of Him, then we cannot lose our first love, which is Jesus.  We can’t lose the One Who we are now part of. The Truth tells us that Jesus will never leave us or forsake us, Hebrews 13:5. The Truth is that salvation is a gift, having Jesus as our first love is a gift from God, Ephesians 2:8. And a-men our Father never takes back His gifts.  

For God's gifts and His call are irrevocable. [He never withdraws them when once they are given, and He does not change His mind about those to whom He gives His grace or to whom He sends His call.] Romans 11:29

This is why the Holy Spirit had Paul write in the Book of Ephesians about our oneness with Jesus. Faith comes from hearing and hearing from the Word of God. The leadership, or messenger, of the church of Ephesus were not one with our Father. They never had received the gift of being one with Him through Christ, so they left Him. Remember Jesus had this letter written to the messenger, or leadership, of the church, not to the children of God within the church.