A few things that convince me that saints of God are preserved by the Grace of God:
1. The Power of God, which is the greatest power (John 10:28-29; I Peter 1:5; Jude 24)
2. The Love of God, which is beyond measure (Rom. 8:28; John 3:16; Eph. 3:18.19)
3. The Immutability of God - He will never change; not His love; not His power; etc. (e.g. Mal. 3:6; Heb. 13:8)
4. The Sufficiency of the shed Blood of Christ (I John 1:7; Rev. 1:5; John 1:29)
5. The Guarantee (Earnest) of the Holy Spirit (Eph. 1:13-14; 4:30; II Cor. 1:22)
6. The Intercession of Jesus Christ (Heb. 7:25; Rom. 8:34)
7. The Predestination of God (Eph. 1:11; Rom. 8:29-39)
There is a song I like that always make me think of preservation by the Grace of God -
No Vacant Seats in Heaven by Mrs. J. B. Edwards (found in The Harp of Ages). Here is the first verse and chorus:
Our hearts are filled with sorrow,
When Jesus calls to claim His own;
A seat is then left vacant,
Yes, vacant in our earthly home.
No vacant seats in Heaven -
No vacant seats around God's throne;
Up there 'tis joy and gladness,
Oh, gloryland, sweet heavenly home.
There will be no vacant seat in Heaven. If God has prepared a seat there, it will be filled not vacant. If it is filled, it will never become vacant at some future time, either.