Quote: The admonition of going into all the world and preaching the gospel was given to the Apostles only.
Technically that would be true. "Go *ye*" In a sense they still are - in the Scriptures.
However, it does not follow that Christians are not to spread the word of God. For example:
Rom 10:14 How then shall they call on him in whom they have not believed? and how shall they believe in him of whom they have not heard? and how shall they hear without a preacher?
Rom 10:15 And how shall they preach, except they be sent? as it is written, How beautiful are the feet of them that preach the gospel of peace, and bring glad tidings of good things!
Without a preacher people would not hear - if they do not hear they cannot believe - if they do not believe they cannot call on the Lord.
The key is "except they be sent". There are thousands who preach who were never sent by God.
What is the test?
1Jn 2:4 He that saith, I know him, and keepeth not his commandments, is a liar, and the truth is not in him.