Mat 23:20 Whoso therefore shall swear by the altar, swears by it, and by all things thereon.

Mat 23:21 And whoso shall swear by the temple, swears by it, and by him that dwells therein.

Mat 23:22 And he that shall swear by heaven, swears by the throne of God, and by him that sits thereon.

Why does Christ go into this level of detail about "swearing," a practice he has already more generally forbidden in Matt: 5:34? More to the point, what does this have to do with the large context here of his criticism of the religious leaders of his time?

As usual, the answer is that this connects Christ's meaning to his larger use of symbols representing the physical, mental, emotional, and spiritual realms. The swearing Christ condemns generally is making oaths to other people involving God. (See posting here.) Here, the discussion is specifically on what we are doing when we make promises directly to God as part of a religious practice, which is very much a part of what Christ recommends as part of the body/mind/relationship/spirit process.

Swearing, as all spoken words, puts thoughts into words. As such, it is part of the mental realm. Christ always tries to connect mental thoughts with physical actions, human relationships, and our spiritual purpose. That is the purpose of these three verses. The physical act of making sacrifices at an altar is connected to the social act of going to the temple, which are both connected to our spiritual purpose: getting in touch with God.

Swearing, the altar, the temple, and heaven are therefore extensions of Christ's use of symbols, becoming symbolic of the mental, physical, emotional, and spiritual realms. They are all connected to each other and are another example of the complete cycle that connects us to God. Our thoughts lead to actions. Our actions create human relationships. Our relationships with each other connects us to our purpose, which is the spirit given us by God.

In the larger topic of religious leaders, Christ sees no role for "leadership" in this process. Promises to God are between one individual and God. The temple is a public forum for displaying that relationships but while making promises in public may have an important role in the process, a religious hierarchy does not.

"Thereon" is from epano, which means "above a place."

"Altar" is from thusiasterion, which means "an altar for slaying sacrifices," "an altar for burnt sacrifices," and "an altar." It is a derivative of thusia, which means "sacrifice" or "victim."