Berkhof:

b. It is a bona fide calling. The external calling is a calling in good faith, a calling that is seriously meant. It is not an invitation coupled with the hope that it will not be accepted. When God calls the sinner to accept Christ by faith, He earnestly desires this; and when He promises those who repent and believe eternal life, His promise is dependable. This follows from the very nature, from the veracity, of God. It is blasphemous to think that God would be guilty of equivocation and deception, that He would say one thing and mean another, that He would earnestly plead with the sinner to repent and believe unto salvation, and at the same time not desire it in any sense of the word. The bona fide character of the external call is proved by the following passages of Scripture: Num. 23:19; Ps. 81:13-16; Prov. 1:24; Isa. 1:18-20; Ezek. 18:23,32; 33:11; Matt. 21:37; II Tim. 2:13. The Canons of Dort also assert it explicitly in III and IV, 8.

Louis Berkhof, Systematic Theology (Grand Rapids: Eerdmans, 1969), 462.

Credit to Tony.