First of all, trinitarians don't always call apostolics a cult. Only some do. And it is not limited to trinitarians. Sometimes other kinds of unblievers use this term against us. The question is, why? One reason is because some people have been taught that any Christian group that rejects the trinity is automatically a cult. To them, rejecting the doctrine of the trinity is equal to rejecting the orthodox faith. This view has been published in a number of books which some trinitarians have read. These people would say the same thing about Jehovah's Witnesses, because they reject the doctrine of the trinity and believe instead in two.
The other reason some people call us a cult is the age-old practice of ad hominem. This is when a person attacks someone personally they perceive as an opponent rather than debating them on logical grounds. In other words, it is name-calling! Jesus warned that we would be hated by all nations 'for His name's sake' (Matthew 24:9). One of the central issues in the debate over Oneness and the trinity is the name of Jesus. It should be of no surprise to us when people who cannot accept that Jesus is the Father end up attacking us and calling us names. After all, they cannot defeat us by a logiocal discussion of the Scriptures.
Today, the word 'cult' conjurs up images of the followers of Jim Jones drinking poison-laced Cool Aid and the followers of David Koresh being burned up by the ATF down in Waco. But this has not always been the case. For example, many historians have refered to the ancient Jewish religion as the 'Temple Cult' and some have refered to early Christianity as the 'Jesus Cult.' In French the word 'cult' means a religion assembly or a church and has no such negative connotations.
The fact is, the people who are saying that you have joined a cult are implying that you have lost your ability to think for yourself and have come under the irresistable sway of some powerful religious leader. Really, they are trying to convince themselves of this so that they can excuse themselves from accepting your message of truth. We encourage you to stand firm. There are many people today who once called apostolics a cult but are now singing the praises of the Lord Jesus. Pray that God will open the eyes of your trinitarian friends.