Charles Hodge has an excellent summary of Romans 8 in his Systematic Theology (III.XVI.8). In this section he gives six arguments of Paul’s proving that those whom Christ has saved will never come into condemnation. In other words, they will be preserved. Here’s Hodge:
The whole of the eighth chapter of the Epistle to the Romans is designed to prove the certain salvation of all who believe. The proposition to be established is, that there is “no condemnation to them which are in Christ Jesus.” That is, they can never perish; they can never be so separated from Christ as to come into condemnation
The Apostle’s first argument to establish that proposition, is, that believers are delivered from the law by the sacrifice of Christ. The believer, therefore, is not under the law which condemns, as Paul had before said in Romans 6:4.
His second argument is that they have already within them the principle of eternal life. That principle is the Spirit of God; “the life-giving” as He was designated by the ancient Church. To be carnally minded is death. To be spiritually minded is life and peace.
The third argument for the security of believers, is, that they are the sons of God. As many as are led by the Spirit of God, they are the sons of God. That
The fourth argument is from the purpose of God. Those whom He has predestinated to be conformed to the image of his Son, them He calls to the exercise of faith and repentance; and whom He thus calls He justifies, He provides for them and imputes to them a righteousness which satisfies the demands of the law, and which entitles them in Christ and for his sake to eternal life; and those whom He justifies He glorifies. There is no flaw in this chain.
Paul’s fifth argument is from the love of God. As stated above, the Apostle argues from the greatness, the freeness, and the immutability of that love that its objects never can be lost.
The sixth argument of the Apostle is that, as the love of God is infinitely great and altogether gratuitous, it is also immutable, and, therefore, believers shall certainly be saved.
The above list has been edited for length. You can find the entire excellent discussion in volume 3 of Hodge’s Systematic Theology.
Covenant Presbyterian Church (OPC)