How did Paul know that? Had he seen the book of life, when caught up to the third heavens? No; but he very plainly and evidently saw the work of God's Spirit upon these persons' souls, by their fruits of faith and labors of love to the glory of the Lord Jesus: hence he speaks so confidently of them. This furnishes us with some glorious truths.
The names of all God's chosen people are registered in the book of life, called the Lamb's book of life. Rev. 21:27. This implies the eternal love of God to them; choice of them, personal knowledge of each of them, value for them, care over them, and their certain enjoyment of God in heaven and glory: for, says the Lord of hosts, "they shall be mine in that day when I make up my jewels." Mal. 3:17. But may not the name of a believer in Christ be blotted out of this book of life and he suffered to perish? No, saith the LAMB, "I will not blot his name out of the book of life." Rev. 3:5. It is his book: they are too dearly loved and too dearly bought, to be blotted out. Not all the powers of sin, earth and hell, can make him a liar: he has said, "I give unto my sheep eternal life; they shall never perish, neither shall any pluck them out of my hand." John 10:28. Say, believer, what hadst thou done to deserve that thy name should be enrolled in the annals of eternal love? What canst thou do to merit its continuance in the book of life? Methinks thy generous, loving soul recoils at these thoughts. Love did the mighty deed: grace reigns: salvation from first to last is all of free-gift to the glory of God.
Those whose names are written in the book of life may attain to a comfortable knowledge of it. How? Not by seeing this book; not by any voice, visions, or fresh revelations from heaven: but says Paul of these, "They labored with me in the Gospel." This implies faith in the Lord Jesus, love to him, delight in him and in the Gospel of his grace. When the Gospel is indeed good news to us miserable sinners; when we see the glory of the Father, Son and Spirit, shine in it; when it captivates our affections, enlivens our hearts, invigorates our minds, animates our hopes, brings peace to our consciences, and subdues the whole man, to give up all for it, and to live under its influence, we may be well assured that our names are written in heaven: for the power of God hath effected this upon our heart, as a consequence of it, and a sure evidence to prove it. "We are chosen from the beginning to salvation, through sanctification of the Spirit and belief of the truth." 2 Thess. 2:13. Without a holy faith in Jesus, a holy life from him, and a holy walk before him, we can enjoy no comfort from electing love. But these are evident tokens of the salvation of God. Phil. 1:25.