There is no expiration date on this promise. It does not merely say, "I will not cast out a sinner at his first coming," but "I will never cast him out." The original reads, "I will not, not cast out," or "I will never, never cast out." The text means that Christ will not at first reject a believer, and that as He will not do it at first, so He will not to the last.
But suppose the believer sins after coming? "If anyone does sin, we have an advocate with the Father, Jesus Christ the righteous." 1 John 2:1 But believers may fall under temptation! "God is faithful, and he will not let you be tempted beyond your ability, but with the temptation he will also provide the way of escape, that you may be able to endure it." 1 Corinthians 10:13 But the believer may fall into sin as David did! Yes, but He will "Purge me with hyssop, and I shall be clean; wash me, and I shall be whiter than snow." Psalm 51:7
Once in Christ, in Christ forever,Nothing from His love can sever.
Jesus said, "I give them eternal life, and they will never perish, and no one will snatch them out of my hand." John 10:28 What do you say to this, O trembling, feeble mind? This is a precious mercy. Coming to Christ, you do not come to One who will treat you well for a little while and then send you about your business, but He will receive you and make you His bride, and you shall be His forever! Live no longer in the spirit of bondage to fear, but in the spirit of adoption, which cries, "Abba, Father!" Oh, the grace of these words: "I will never cast out."
(Taken from Morning and Evening by C.H. Spurgeon)