Then the Lord appeared to Abram and said, "To your offspring I will give this land." So he built there an altar to the Lord, who had appeared to him. - Genesis 12 v 7
God showing up in Abraham's (Or Abram as we at this point) was a huge thing. This was no voice from heaven this was God showing up and giving him a promise in person. This was something that he always wanted to be fresh in his memory and the memory of others. So he builds an altar to remember.
I've never had Jesus stand in front of me and promise me anything but I know that in many situations and circumstances he has shown up for me through His Holy Spirit.
A Word of comfort in times of tragedy.
A Word of faith in times of doubt.
A Word of encouragement in times of doubt.
A Word of direction in times of confusion.
But too often I move on and forget too quickly.
The Hebrew word for Altar is "mizbe'ah", from a word meaning "to slay".
So Abraham was building a place of sacrifice and remembrance.
Paul carries this on in his letter to the church in Rome:
I appeal to you therefore, brothers, by the mercies of God, to present your bodies as a living sacrifice, holy and acceptable to God, which is your spiritual worship. - Romans 12 v 1
For every time you have shown up in my life God, I say a humble thank you, help me never to forget your unbelievable mercy and grace that you have shown me and I put myself on the alter to die to myself again so I can find abundant life in You.