en_tn_condensed/mat/05/35.md

swear not at all ... city of the great King

Here Jesus means that when people make a promise or when they say that something is true, they must not swear by anything. Some people were teaching that if a person swears by God that he will do something, then he must do it, but if he swears by something else, such as by heaven or earth, then it is less offensive if he does not do what he swore to do. Jesus says that swearing by heaven or earth or Jerusalem is just as serious as swearing by God because those things all belong to God.

it is the footstool for his feet

This metaphor means the earth also belongs to God. Alternate translation: "it is like a footstool where a king rests his feet" (See: rc://en/ta/man/translate/figs-metaphor)

for it is the city of the great King

"for it is the city that belongs to God, the great King"