So, you exclude the General Baptists and Freewill Baptists from being NT churches?
Yes, because Free Will Baptists believe one can lose their salvation.
John 1:11-13: He came to his own, and his own did not receive im. But as many as received him, to them he gave the right to become children of God, to those who believe in his name; who were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God.
John 5:24: Verily, verily I say unto you, he that hears my word, and believes on him that sent me, has everlasting life, and shall not come into condemnation; but is passed from death unto life.
John 6:39-40, 44: And this is the Father's will which has sent me, that of all which he has given me I should lose nothing, but should raise it up again at the last day. And this is the will of him that sent me, that every one which sees the Son, and believes on him, may have everlasting life; and I will raise him up at the last day... No one can come to me unless the Father who sent me draws him; and I will raise him up at the last day.
John 10:27-30: My sheep listen to my voice; I know them, and they follow me. I give them eternal life, and they shall never perish; no one can snatch them out of my hand. My Father, who has given them to me, is greater than all; no one can snatch them out of my Father's hand. I and the Father are one.
Romans 8:29-30: For whom he foreknew, he also foreordained to be conformed to the image of his Son, that he might be the firstborn among many brothers; and whom he foreordained, them he also called; and whom he called, them he also justified; and whom he justified, them he also glorified.
Ephesians 1:4,5,11: For he chose us in him before the creation of the world to be holy and blameless in his sight, having foreordained us unto adoption as sons through Jesus Christ unto himself, according to the good pleasure of his will...
1 John 3:14: We know that we have passed from death unto life.
1 John 5:13: These things have I written unto you that believe on the name of the Son of God; that you may know that you have eternal life, and that you may beleive on the name of the Son of God.
Also, I would include Primitive Baptists, as they ignore the Great Commission. Whether or not you believe in Doctrines of Grace, we are commanded by the Lord to tell the Good News.
I want to make one thing clear, the op is about the definition of NT churches. This in no way means that members of these churches are not saved. There are saved primitives, general, southern, and free will Baptists. There are saved members of all Protestant denominations. There are saved Catholics despite their church.