Surah At-Tawba 9:10

They have purchased with the Ayat of Allah a little gain, and they hindered men from His way; evil indeed is that which they used to do (9)With regard to a believer, they respect not the ties, either of kinship or of covenant! It is they who are the transgressors (10)But if they repent, perform the Salah and give the Zakah, then they are your brethren in religion. (In this way) We explain the Ayat in detail for a people who know (11)

Allah admonishes the idolators and encourages the believers to fight against them because,

اشْتَرَوْا بِآيَاتِ اللَّهِ ثَمَنًا قَلِيلًا

(They have purchased with the Ayat of Allah a little gain,) idolators exchanged following the Ayat of Allah with the lower affairs of life that they indulged in,

فَصَدُّوا عَن سَبِيلِهِ

(and they hindered men from His way), trying to prevent the believers from following the truth,

إِنَّهُمْ سَاءَ مَا كَانُوا يَعْمَلُونَ - لَا يَرْقُبُونَ فِي مُؤْمِنٍ إِلًّا وَلَا ذِمَّةً

(evil indeed is that which they used to do. With regard to a believer, they respect not the ties, either of kinship or of covenant!)[9:9-10].

We explained these meanings before, as well as, the meaning of,

فَإِن تَابُوا وَأَقَامُوا الصَّلَاةَ

(But if they repent, perform the Salah...)