Numbers 18:21
And, behold, I have given the children of Levi all the tenth in Israel for an inheritance, for their service which they serve, even the service of the tabernacle of the congregation. And, behold, I have given the children of Levi all the tenth in Israel for an inheritance, for their service which they serve, even the service of the tabernacle of the congregation.
The whole tenth in Israel was given to the children of Levi as their inheritance in return for their service in the tabernacle of the congregation, and this is the first tithe. The tithe is the Lord's own provision assigned for the specific service He has specified. It is to be faithfully paid and brought into the treasury for that appointed use.
It is not to be appropriated by the church for caring for poor, sick, or aged people. That care remains a responsibility of the church, but it is separate from the tithe's appointed use. The tithe supports those who serve in sacred work by preaching the Word, including helping prepare them for ministry. It also supports those who teach God's word and educate students in spiritual matters.