Those who honor me I will honor, but those who despise me will be disdained

1 Samuel 2:27-36
Now a man of God came to Eli and said to him, “This is what the Lord says: ‘Did I not clearly reveal myself to your ancestor’s family when they were in Egypt under Pharaoh? Why do you scorn my sacrifice and offering that I prescribed for my dwelling? Why do you honor your sons more than me by fattening yourselves on the choice parts of every offering made by my people Israel?’
Far be it from me! Those who honor me I will honor, but those who despise me will be disdained.
The time is coming when I will cut short your strength and the strength of your priestly house, so that no one in it will reach old age,... ‘And what happens to your two sons, Hophni and Phinehas, will be a sign to you—they will both die on the same day. 
I will raise up for myself a faithful priest, who will do according to what is in my heart and mind. I will firmly establish his priestly house, and they will minister before my anointed one always.
The prophet's prediction is fulfilled in the death of Eli's sons in battle at Aphek. The priesthood passed from Eli's family to the line of Zadok in David's day.
It was the will of the Lord to slay the,: The writer puts it this way because God is sovereign in every circumstance. It is equally true that their death is a direct outcome of their own free choice to disobey God. The Bible sees no conflict between God's sovereignty and our free will.
Why?
God sent a man to warn Eli, that his family will not be a priest house anymore. For Eli honor his sons more than God.
Those who honor me I will honor, but those who despise me will be disdained. This is the rule before the Lord. No one can escape from the anger of the Lord.
The punishment of the Lord to Eli's is very severe and up to many generations.
The faithful priest who will do according to what is in God's heart and mind.
How?
Be a faithful priest follow and meditate God's will and God's word.