Allah is informing us of the honor of Ibrahim Al-Khalil, who He made an Imam for the people, and a model to be imitated, because of the way he conducted himself and adhered to Tawhid. This honor was given to Prophet Ibrahim when he adhered to Allah's decisions and prohibitions. This is why Allah said,
﴿وَإِذِ ابْتَلَى إِبْرَهِيمَ رَبُّهُ بِكَلِمَـتٍ﴾
(And (remember) when the Lord of Ibrahim (i.e., Allah) tried him with (certain) commands).
This Ayah means, O Muhammad! Remind the idolators and the People of the Scriptures, who pretend to be followers of the religion of Ibrahim, while in reality they do not follow it, while you, O Muhammad, and your followers are the true followers of his religion; remind them of the commands and prohibitions that Allah tested Ibrahim with.
(which he fulfilled.) indicating that Ibrahim implemented all of Allah's orders. Allah said in another Ayah,
﴿وَإِبْرَهِيمَ الَّذِى وَفَّى ﴾
(And of Ibrahim (Abraham) who fulfilled (or conveyed) all that (Allah ordered him to do or convey)) (53:37)
meaning, he was truthful and he was obedient to Allah's legislation. Also, Allah said,
﴿إِنَّ إِبْرَهِيمَ كَانَ أُمَّةً قَـنِتًا لِلَّهِ حَنِيفًا وَلَمْ يَكُ مِنَ الْمُشْرِكِينَ - شَاكِراً لانْعُمِهِ اجْتَبَـهُ وَهَدَاهُ إِلَى صِرَطٍ مُّسْتَقِيمٍ - وَءاتَيْنَـهُ فِى الْدُّنْيَا حَسَنَةً وَإِنَّهُ فِى الاٌّخِرَةِ لَمِنَ الصَّـلِحِينَ - ثُمَّ أَوْحَيْنَآ إِلَيْكَ أَنِ اتَّبِعْ مِلَّةَ إِبْرَهِيمَ حَنِيفًا وَمَا كَانَ مِنَ الْمُشْرِكِينَ ﴾
(Verily, Ibrahim was an Ummah (or a nation), obedient to Allah, Hanif (i.e. to worship none but Allah), and he was not one of those who were Al-Mushrikin (polytheists), (He was) thankful for His (Allah's) favors. He (Allah) chose him and guided him to a straight path. And We gave him good in this world, and in the Hereafter he shall be of the righteous. Then, We have sent the revelation to you (O Muhammad saying): "Follow the religion of Ibrahim Hanif (Islamic Monotheism ـ to worship none but Allah) and he was not of the Mushrikin.) (16:120-123)
﴿قُلْ إِنَّنِى هَدَانِى رَبِّى إِلَى صِرَطٍ مُّسْتَقِيمٍ دِينًا قِيَمًا مِّلَّةَ إِبْرَاهِيمَ حَنِيفًا وَمَا كَانَ مِنَ الْمُشْرِكِينَ ﴾
(Say (O Muhammad ): "Truly, my Lord has guided me to a straight path, a right religion, the religion of Ibrahim, Hanifan, and Ibrahim (to worship none but Allah, alone) and he was not of Al-Mushrikin.'') (6:161) and,
﴿مَا كَانَ إِبْرَهِيمُ يَهُودِيًّا وَلاَ نَصْرَانِيًّا وَلَكِن كَانَ حَنِيفًا مُّسْلِمًا وَمَا كَانَ مِنَ الْمُشْرِكِينَ - إِنَّ أَوْلَى النَّاسِ بِإِبْرَهِيمَ لَلَّذِينَ اتَّبَعُوهُ وَهَـذَا النَّبِىُّ وَالَّذِينَ ءَامَنُواْ وَاللَّهُ وَلِىُّ الْمُؤْمِنِينَ ﴾
(Ibrahim was neither a Jew nor a Christian, but he was a true Muslim Hanifan (Islamic Monotheism ـ to worship none but Allah alone) and he was not of Al-Mushrikin. Verily, among mankind who have the best claim to Ibrahim are those who followed him, and this Prophet (Muhammad ) and those who have believed (Muslims). And Allah is the Wali (Protector and Helper) of the believers) (3:67-68).
(with Kalimat (words)) which means, "Laws, commandments and prohibitions.'' `Words' as mentioned here, sometimes refers to what Allah has willed, such as Allah's statement about Maryam,
﴿وَصَدَّقَتْ بِكَلِمَـتِ رَبَّهَا وَكُتُبِهِ وَكَانَتْ مِنَ الْقَـنِتِينَ﴾
(And she testified to the truth of the Words of her Lord, and (also believed in) His Scriptures, and she was of the Qanitin (i.e. obedient to Allah)) (66:12).
"Words'' also refers to Allah's Law, such as Allah's statement,
﴿وَتَمَّتْ كَلِمَةُ رَبِّكَ صِدْقاً وَعَدْلاً﴾
(And the Word of your Lord has been fulfilled in truth and in justice) (6:115) meaning, His legislation. "Words'' also means truthful news, or a just commandment or prohibition. For instance, Allah said,
﴿وَإِذِ ابْتَلَى إِبْرَهِيمَ رَبُّهُ بِكَلِمَـتٍ فَأَتَمَّهُنَّ﴾
(And (remember) when the Lord of Ibrahim tried him with (certain) Words (commands), which he fulfilled) meaning, he adhered to them, Allah said,
﴿إِنِّى جَـعِلُكَ لِلنَّاسِ إِمَامًا﴾
("Verily, I am going to make you an Imam (a leader) for mankind (to follow you).'') as a reward for Ibrahim's good deeds, adhering to the commandments and avoiding the prohibitions. This is why Allah made Ibrahim a role model for the people, and an Imam whose conduct and path are imitated and followed.