When the ayah {O you wrapped up [in a mantle]!} [Surah Al-Muddathir: 1] was revealed, this was an announcement from Allah (Glorified is He) to the Prophet (peace be upon him) for him to start calling people to Islam.
Therefore, the Prophet (peace be upon him) started to call people to Allah in secret. This continued for the first three years of the call. The call was in secret in order to protect the call and the Muslims who followed it, which were only a small number at that time. This was in order for the polytheists not to find out about the call and so overcome it when it was still in its cradle.
In this period, the Prophet (peace be upon him) used to call those he trusted, those who he thought would accept the religion and would keep matters secret. This was an islamically accepted way to deal with such a situation; the interests of the mission needed this precaution at such a time.