Altars were a vital part of worship in the Old Testatment. It was usually built to offer sacrifices and offerings to the Lord. For this purpose, let's define the altar as a contact point between God and man. When God appeared to Manoah, the angel ascended in the flame of the sacrifice.
In Elijah's time, the Israelites forsook God for Baal and other gods; threw down the altars and killed the prophets of God. They backslid. But when it was time to restore them back to God, the first thing Elijah did was to repair the altar.
Many of us in the faith have lost our spiritual connection to God (even though we might still be attending church!). We have accepted strange doctrines and now endorse abominable practices. We no longer do those things we used to do when we first believed; in other words, we have left our first love. Some of us no longer pray and study the word as we used to, we are no longer involved in evangelism, we've lost our passion for the house of God and other christian meetings, we no longer give or care for others as we once did and we are even now living in sin.
If you're in such condition, it's time to repair your altar. Revelations 2:5 says, "Consider how far you have fallen! Repent and do the things you did at first. If you do not repent, I will come to you and remove your lampstand from its place." I pray that God grants us the grace to finish strong. Amen.