When we say that the Security of the Believer is explicitly taught in the Scriptures, we are saying that it is definitely, definitively, expressly, specifically, unambiguously and unequivocally taught in the Bible.
Why is the explicit Security of the Believer a part of the defense of the Gospel? If the Gospel places the responsibility of saving man on the finished work of Jesus Christ - that is, if salvation is a work of God, then like all the works of God, He alone must finish His work.