Numbers 18:19
All the heave offerings of the holy things, which the children of Israel offer unto the LORD, have I given thee, and thy sons and thy daughters with thee, by a statute for ever: it is a covenant of salt for ever before the LORD unto thee and to thy seed with thee.
Numbers 16-19 form a solemn and sad record of sin, with God's object-lessons to teach the true way. God set apart the tribe of Levi from the other tribes for the service of the tabernacle, and He provided for His ministers by giving them the tithes of Israel to inherit. This was because they were to have no inheritance in the land, since the tithe is the Lord's and holy unto Him. The holy heave offerings were given to Aaron and to his sons and his daughters with him as a lasting provision established as a permanent covenant before the Lord.
Those portions were the parts of Israel's holy offerings not consumed by fire, including the reserved parts of sacrifices and the best of the oil and wine and wheat. The covenant concerning the offerings was ratified by the use of salt according to God's direction, for the law required salt to be offered with all offerings. More specifically, He required salt in all the meat-offerings made to Him, while strictly forbidding leaven in them. A covenant of salt is a solemn, lasting, permanent, incorruptible and unbreakable bond, an everlasting covenant with God.
The offerings seasoned with salt bound the people to Jehovah, and when the priests ate the offerings they entered into covenant with God and pledged faithful allegiance and persistent service. Salt is an element of preserving power put on foods to give them relish and to preserve them from putrefaction and decay, while the pervading and transforming effect of leaven hastens decay, so salt and leaven picture opposite minds and characters. Salt signifies harmony of thought, feeling, and purpose, stable friendship, union, and peace, and it stands for the saving covenant, saving grace, the saving qualities of Christ's righteousness, and the grace of God in the heart which gives relish, preserves from corruption, and strengthens the bond of union. God's people are the salt of the earth and, as Christ's representatives, are His covenant with its inhabitants. Jesus is the Head of this eternal covenant and the great Bread-offering given for humanity, and those who offer themselves to God are acceptable through the power of the Spirit; seasoning the sacrifice of self with God's grace binds them to the Lord in an indissoluble covenant and assures them of His presence and eternal salvation, which explains the salt spoken of in Mark 9:49.