The bible talks about it various times in the Bible that who is divided will fall and is sinning against the true church of Christ. Satan is the one responsible to have caused divisions in his church that was established on Peter.
10 If people are causing divisions among you, give a first and second warning. After that, have nothing more to do with them. 11 For people like that have turned away from the truth, and their own sins condemn them.
Titus 3:10-11
Being a sectarian is to be divided. Therefore, you have turned away from the truth and from the true church of Christ.
10 I appeal to you, dear brothers and sisters,[c] by the authority of our Lord Jesus Christ, to live in harmony with each other. Let there be no divisions in the church. Rather, be of one mind, united in thought and purpose.
Passage 1 Corinthians 1 :10
To formally be part of another church is already a sin because the truth and sacraments that the catholic church has is not present there. They are merely doctrines and traditions that men made up as to make money or business. Satan utilized men as to divide the true church of Christ such as Luther the founder of the Angelican church who rebelled against the Catholic church on holloween to later regret it and ask a priest to confess him before he died. Later on, more men became used by Satan and more sectarian religions appeared which branched off from the Catholic Church. They are nothing more but rebellions that were produced by Satan.
Jesus in the begenning always wanted a united church that would be one with him:
20 “I am praying not only for these disciples but also for all who will ever believe in me through their message. 21 I pray that they will all be one, just as you and I are one—as you are in me, Father, and I am in you.
Passage John 17:20-21
We are all supposed to be one and not divided up into 30,000 sectarian religions that are all based or came from the Catholic Church.