Becoming the first follower of Muhammad
According to the traditional Sunni narrative, when Muhammad reported his first revelation from the Angel Gabriel (Jibril), Khadija was the first person to accept Al-Haqq The Truth i.e. she accepted Islam. After his experience in the cave of Hira, Muhammad returned home to Khadija in a state of terror, pleading for her to cover him with a blanket. After calming down, he described the encounter to Khadija, who comforted him with the words that Allah would surely protect him from any danger, and would never allow anyone to revile him as he was a man of peace and reconciliation and always extended the hand of friendship to all. According to some sources, it was Khadija's cousin, Waraqah ibn Nawfal, who confirmed Muhammad's prophethood soon afterwards.
Yahya ibn `Afeef is quoted saying that he once came, during the period of Jahiliyyah (before the advent of Islam), to Mecca to be hosted by 'Abbas ibn 'Abd al-Muttalib, one of Muhammad's uncles mentioned above. 'When the sun started rising', he said, 'I saw a man who came out of a place not far from us, faced the Kaaba and started performing his prayers. He hardly started before being joined by a young boy who stood on his right side, then by a woman who stood behind them. When he bowed down, the young boy and the woman bowed, and when he stood up straight, they, too, did likewise. When he prostrated, they, too, prostrated.' He expressed his amazement at that, saying to Abbas: 'This is quite strange, O Abbas!' 'Is it, really?' responded al-Abbas. 'Do you know who he is?' Abbas asked his guest, who answered in the negative. 'He is Muhammad ibn Abdullah, my nephew. Do you know who the young boy is?' he asked again. 'No, indeed', answered the guest. 'He is Ali son of Abu Talib. Do you know who the woman is?' The answer came again in the negative, to which Abbas said, 'She is Khadija bint Khuwaylid, my nephew's wife.' This incident is included in the books of both Ahmad ibn Hanbal and Al-Tirmidhi, each detailing it in his own Ṣaḥīḥ.
Khadija was supportive of Muhammad's prophetic mission, always helping in his work, proclaiming his message and belittling any opposition to his prophecies. It was her encouragement that helped Muhammad believe in his mission and spread Islam. Khadija also invested her wealth in the mission. When the polytheists and aristocrats of the Quraysh harassed the Muslims, she used her money to ransom Muslim slaves and feed the Muslim community.
In 616, the Quraysh declared a trade boycott against the Hashim clan. They attacked, imprisoned and beat the Muslims, who sometimes went for days without food or drink. Khadija continued to maintain the community until the boycott was lifted in late 619 or early 620.