On pages 250-251 of his text, "Biblical Separation" , Dr. Pickering quotes several of his opponents to this teaching, those who admit for example, "The scriptures say nothing about secondary separationism," and, "There is no such thing as first-degree, second-degree separation, etc." Pickering acknowledges that secondary separationism is not labeled specifically as such, in the scriptures. But that's not the end of it: Pickering also has to justify his teaching past this confession that it is no where directly taught in scripture.
Pickering insists that the practice can be drawn as a valid principle from the following scripture:
Now we command you, brethren, in the name of our Lord Jesus Christ, that ye withdraw yourselves from every brother that walketh disorderly, and not after the tradition which he received from us.... And if any man obey not our word by this epistle, note that man, and have no company with him, that he may be ashamed" (2 thess 3:6, 14).
Pickering acknowledges that in context the Thessalonian Christians were merely shirking their responsibilities and had become busybodies. He then immediately rationalizes upon this scripture, saying (on page 255): "The question is this: Is that all this passage teaches? Does the principle go beyond the specific situation evident at Thessalonica, and should it be applied in other situations as well?" He then moves right in to pointing out Paul had authority from God to teach the truth. And he points out that those idle brothers, in their disregard of Paul, were called "disorderly." Through this simple but poor connection, Pickering gives his readers the impression that 2 Thess. 3 is calling all those who reject Paul's written authority, those who are idle.
Rather, I ask you to take a look at this passage and see if it is not saying that those who Paul is calling "disorderly" are those practicing idleness. It is the demonstration of idleness, which Paul is commanding the Thessalonicans to take note of, and withhold their company from.
Pickering nevertheless insists that "Their improper actions resulted from their incorrect grasp of doctrine." Friends, this next unabridged paragraph (pg. 256-257) is where you are going to see the door open up for a moral kind of legalism in the practice of secondary separationism....
The main question among separatists is this: Does this portion of scripture apply only and exclusively to people who leave their work, sponge off other believers, and live idle lives that contribute to spiritual confusion? Certainly this is the context of the passage as it is written. Is it not true, however, that the principle clearly seen here applies to other situations? The principle is this: When our brethren do things that are wrong - caused by an incomplete knowledge of or deliberate disobedience to some teaching of Scripture - we should not merely continue to fellowship with them as those who have done nothing wrong.... It is the principle of maintaining a pure walk that should be stressed. It is the principle of refusing to condone, honor, or utilize those who continually and knowingly are following a course of action that is harmful to other believers and to the welfare of the churches.
Can he defend his interpretation with other scriptures? He had none others. Following this look into 2 Thess. 3, Pickering quotes the commentary of other respected fundamentalists who happen to agree about this interpretation from the text, and concludes, "We cite them, however, to show that there have been a host of men through the years who have believed that the practice of so-called secondary separation has Biblical support" (pg. 261).
 Pickering, Ernest & Houghton, Myron. Biblical Separation: The Struggle for a Pure Church. Schaumberg, Illinois: Regular Baptist Press; 2008.