Genesis13:18 Then removed his tent, and came and dwelt in the plain of Mamre, which is in Hebron, and built there an altar unto the Lord.
After Abram and Lot separated, the Lord appeared again to him and Blessed him again by repeating The Promise of blessing and seed and land. Abram’s response to the Lord’s reaffirmation of His Covenant Promise was to believe God. He moved his tent to the center of the Land of Promise, and there he built an altar unto the Lord.
To build an altar means to thank God for His promise and to worship Him, but especially, to believe the Lord and receive His Word. He did not build the altar to try to please God and appease Him in order to get something from God, that is, to try and move the god to do something good for him. He did not use prayer and devotion, or even sacrifice, to manipulate God. Abram built the altar after the Lord appeared to him, spoke a word of promise, made a unilateral covenant, and did something good.
This is always our response to the Gospel: we believe: we receive the covenant blessings and the gift of eternal life; we worship: we praise Him as our only God and our only good; we thank: since the Lord does it all, the only thing left for us to do is live lives of thanksgiving in every attitude, word, thought, and behavior; we sacrifice: we gladly give up our own time, effort, resources, and lives in the service of others. (“God does need your service, but others do.” [Luther].)
We respond to the Gospel Promise by ”building an altar” wherever we are and wherever we go. John 4:23: ”True worshipers will worship the Father in spirit and truth.”