After identifying Himself to the Ephesians, Jesus begins His letter by informing the church that He knew their works, both collectively and individually (Revelation 2:2-3). He knew their attitudes, thoughts, desires, goals - everything about them! He knew their hearts better than they did, just as He knows ours today.
He tells them He was aware of everything they had gone through - how they had endured much persecution, suffering, and agony because of apostasy and intolerance. He knew of their fortitude in standing for the truth and what a labor it was to persevere, though they never gave in to weariness.
He then mentions how they had rejected false leaders and their subversions. They would not listen to those who tried to pervert the truth and change doctrine, and they stood fast in opposing them once they found such to be liars. He knew how hard they tried to keep the laws and principles of the truth the true apostles had taught them.
In this description, as well as from the history of the first-century church from the book of Acts and the epistles of the apostles, we see a church that had fallen apart despite the strenuous efforts of some members to hang on to the truth. It was a church that had let something vital slip out of its grasp amidst the mounting trials and persecutions of the times.
Christ brings to their attention that they had lost their first love, the ardent desire to please God (verse 4). Their focus had shifted from where it should have been to the problems and the events happening around them. The strife generated by angry words, bad attitudes, friends and family leaving their fellowship, and teachings being changed took its toll on everyone. The byproducts that such turmoil produced were mistrust and suspicion.
Humanly, we might think that God would consider the Ephesians' efforts to hang on to the truth against the apostasy as sufficient, but it is obvious that He does not. For our eternal good, He expects more of us. However, He does not leave us without guidance about how to recapture what we have lost.
Why is first love so important to God? First love is the purest kind of spiritual love we as humans can demonstrate. It is a love that truly shows one's heart is completely given to God. What does first love produce? Paul answers in II Thessalonians 1:3:
We are bound to thank God always for you, brethren, as it is fitting, because your faith grows exceedingly, and the love of every one of you all abounds toward each other.
He also writes in Hebrews 6:10,
For God is not unjust to forget your work and labor of love which you have shown toward His name, in that you have ministered to the saints, and do minister.
True love of God always promotes love of the brethren and love toward fellow man.