"But did I hear you whisper that this was not a convenient time? Then what must I say to you? When will that convenient time come? Shall it come when you are in hell? Will that time be convenient? ... Remember, I have no authority to ask you to come to Christ to-morrow. The Master has given you no invitation to come to him next Tuesday. The invitation is, 'To-day if ye will hear his voice, harden not your hearts as in the provocation,' for the Spirit saith 'to-day.' 'Come now and let us reason together;' why should you put it off? It may be the last warning you shall ever have. Put it off, and you may never weep again in chapel." - From Spurgeon's Sermon Compel Them to Come In
Further recommended reading & listening:
The Best of The Daily Spurgeon
The Forgotten Spurgeon, by Iain Murray