ולמדתם אתם את בניכם לדבר בם .... (יא-יט)
Chazal tell us: “Since the destruction of the Bais HaMikdash, the only thing Hashem has left in this world is the four amos of Halacha.” (ברכות ח.) The Chasam Sofer zt”l writes that this refers to the four levels that man should strive to attain in this world - "ללמוד, וללמד, לשמור, ולעשות" - To learn, teach and safeguard the Torah, and to do the mitzvos. These are the four measurements of halacha. Halacha means to go (הליכה), to move forward, and THIS is what Hashem has in this world. The people who are making strides in Torah learning and observance - there is nothing else that Hashem has that is valuable.
Sefer Mishna Sachir offers a slightly different twist to the words of this Gemara. He quotes the posukim in Krias Shema where we are told to place the mitzvos “on our hearts and tie them on our hands” - that is the first level, a reference to man’s obligation to improve himself. "ולמדתם אותם את בניכם" - “And you shall teach it to your children” - this is the second level, man’s obligation to teach his children the laws of the Torah. "וכתבתם על מזוזת ביתך" - “And you shall write them on the doorposts of your home” - this is the third level, where man must instruct all the members of his household and those that come to his door to observe the Torah. "ובשעריך" - “and your gates” - this is the fourth level which implies how a person must be an example to anyone that he comes in contact with. When a person acts in this manner, and strives to uphold the four levels of HALACHA - of moving forward in the spiritual spheres, then he is creating a substitute for the Bais HaMikdash in this world.
This idea is alluded to in the words: "למען ירבו ימיכם ... כימי השמים על הארץ" - “So that you may increase your days .... like the days of Heaven on this earth.” "כימי השמים" refers to the heavenly kedusha of the Bais HaMikdash, "על הארץ" here in this world! It is truly our obligation to create a place for Hashem to dwell in this world by keeping the four amos of halacha!