It's a figure of speech indicating the severity of the error of the Pharisees. Jesus was calling the Pharisees "sons of hell". The only thing they had going for them was that they had access to the truth but suppressed it in unrighteousness. Their disciples from among the gentiles had no access to the truth, only to the traditions of the Pharisees. Ultimately the Pharisees will be held accountable for their unrighteous proselytizing.
"My brethren, let not many of you become teachers, knowing that we shall receive a stricter judgment." (James 3:1)
Bunker says:
"There are two gospels abroad in the world today. The “Free Will” gospel exalts man and debases God’s salvation. It seeks God out of the flesh, and refuses Him his due. The “Sovereign Grace” Gospel (the only True Gospel), seeks God aright, and bestows upon God all the credit and the glory for the new birth. I did seek God. But I did not seek Him while I was dead. I sought Him after He quickened my spirit, and enlightened mine eyes, and drew me inexorably to Himself. Not of the will of man, but of God. That’s what your Bible says."
"The false “Free Will” gospel has many adherents and many missionaries, most of whom will never question themselves or even stop to think that they are creating children that are TWICE the children of hell than they themselves are. Think about that. Most of these missionaries (just like many of you) are suffering from “I Disease”. I hear from these people every day, “I just want to put God first in my life”, they say; then listen to them: “I was saved when I saw that my life was going bad and I decided to go to church”, or, “I decided to go to a revival”, and “I saw that I was a sinner, and I needed to be saved, so I said the ‘sinner’s prayer” and I went forward to confess this to my priest (pastor), and then I got saved, and now I want to live MY life for God.” All that and they want to put GOD first in their life? How man “I’s” did it take to get to God? You’ve got “I Disease” out there, and it is communicable, and people are spreading it all over the world. Jesus looks at it totally differently:"
I understand the reference to Matt. 23:15, however that allusion is a poor slander of the work of Evangelists like Billy Graham, Luis Palau and Franklin Graham or missionaries under C&MA, AIM or CIM or student ministries like Inter Varsity or Campus Crusade. The Pharisees were directly opposing the word of Jesus. These ministries are advocating and promoting salvation by grace through faith. To say that those who have trusted Christ for their salvation, thinking they made the decision are “children that are TWICE the children of hell” because they have responded to a call to make a decision is a gross misreading of the doctrine of election. My point is the elect are never lost nor can they lose their salvation whether they respond to a call, come to an understanding over a period of time, or have a “Damascus Road” experience or think they made the decision of their own free will. God calls them where they are and in the circumstances they are in according to the understanding the Holy Spirit gives them at the time.
As far as Baptists requiring or promoting a “point in time” experience for baptism, I have never seen that as a requirement in any Baptist constitution nor have I ever heard of it as a requirement in any Baptist church that I have represented. What is required is a clear testimony that they are trusting Christ and only Christ for their salvation. Because it is “Believer’s Baptism” for the most part the candidates are adults that have come to an understanding of their need of salvation later in life so can point to a time when the Holy Spirit convicted them of the need but there are testimonies of those raised in believing homes who cannot point to a definite time but know they are trusting Christ for their salvation and are living that faith.