As an evangelical Protestant, I often listened to my pastor--an ex-Catholic layman--and numerous radio preachers give assurances that heaven was absolutely guaranteed to all who are truly "born again" (Jn. 3:5). Contrary to this belief is the Catholic dogma of perseverance, which is that Christians must voluntarily persevere in the grace of God to the end of their earthly lives in order to see God "face to face" (cf. Catechism of the Catholic Church (CCC) nos. 162, 2016).  Inseparable from this dogma is the truth that Christians can freely choose to reject God's grace, and thus merit eternal condemnation (cf. CCC nos. 1036, 1861).
Protestants such as my former pastor reject these truths because of theological misconceptions. These are reflected in the following questions, often rhetorically posed as supposedly irrefutable objections to the possibility of "losing one's salvation":
• "How can the eternal life that Christians receive when they are born again ever end?"
• "How can a Christian be 'unborn' after being born again in the Spirit?"
• "How can God remain faithful if He can 'renege' on His promise of salvation to all who have believed in Jesus as Lord and Savior?"
These objections reflect misconceptions, which can be corrected through proper understanding of the Catholic doctrine of salvation as a relationship with God through the grace of participating in His eternal life. Examples of apologetics for this doctrine are given at the conclusion of this article. However, effectively sharing this truth requires insight into how the dogma of perseverance is related to the dogma of the Blessed Trinity, the foundation of "the hierarchy of truths" in the Catholic faith. This article attempts to help provide this insight.
Trinitarian Apologetics and the Hierarchy of Truths
Vatican II taught that while Catholics share their faith with separated brethren, they should recall "that in Catholic doctrine there exists a 'hierarchy' of truths . . ." (Decree on Ecumenism, no. 11). The truths in question are all and only the revealed truths contained in the deposit of faith (i.e., the dogmas), which as such must be equally believed with divine faith (cf. Mystery of the Church, no. 4). The hierarchy refers to, on one hand, the dependence of dogmas that are less foundational--but not less certain--upon more foundational dogmas; on the other hand, to the illumination of more foundational dogmas by the less foundational dogmas (cf. Mystery of the Church, no. 4).
The ultimate foundation of the hierarchy of dogmatic truths is the dogma concerning the central mystery of Christian faith, the Blessed Trinity (cf. Catechism no. 234). This foundation is the "common ground" of ecumenical dialogue, since profession of the triune God as revealed in Jesus Christ is the "common denominator" of all genuinely Christian confessions of faith (cf. Decree on Ecumenism, nos. 1, 12, 20). For this reason, ecumenical apologetics should refer to the hierarchy of truth in order to relate Catholic dogmas to the revelation of the triune God.
Obstacles to Trinitarian Apologetics
For the sake of genuine ecumenical progress, it is vitally important that Catholics effectively demonstrate the Trinitarian foundation of dogmas that are rejected by separated brethren, such as the dogma of perseverance. However, while believing Protestants fervently profess the fact of the Trinity, they are either unaware of Trinitarian dogma, or else hold these dogmas in theological isolation from other doctrines, especially in the area of soteriology (i.e., the nature and means of salvation). Non-creedal Protestant communities, in particular "non-denominational" fundamentalist and evangelical churches, are largely ignorant of the real relationships implied in Jesus' revelation of God's Trinitarian name (cf. Matt. 28:19). Indeed, Protestants in such communities are not likely to recognize denial of essential relationships between the divine Persons as heterodox.  In contrast, "classical" Protestant denominations, having retained the Nicene Creed, are at least aware of the real Trinitarian relationships. However, in keeping with Reformation "tradition," these denominations tend to isolate their understanding of salvation from their orthodox profession of the Trinity.
The effect of the isolation of soteriology from Trinitarian dogma is most evident in the pervasive Protestant doctrine of "the eternal security of the believer," also known as "once saved, always saved." According to this doctrine, continual relationship with God is merely, albeit necessarily, symptomatic of salvation. This notion is contrary to the Catholic understanding of salvation as identical to perpetual relationship with God, through participation in His eternally relational life as Father, Son and Holy Spirit.
Eternal security proposes a radical separation of relationship with God from salvation by God, as is evident in the homily "sin cuts Christians off from fellowship with God, but never from the eternal gift of His salvation." This homily at least trivializes the essential message of the Gospel, which is salvation through reconciliation to God (cf. Rom. 5:10-11; 2 Cor. 5:17-20). For Protestants to whom the Gospel is the message of eternal security, salvation through reconciliation with God is strictly instantaneous, since the second birth itself is the unrepeatable initiation into the grace of God through faith in Jesus Christ. The Catholic faith also teaches the second birth to be the definitive and unrepeatable initiation into God's saving grace (cf. Catechism no. 1272). However, in Catholic soteriology the initiation of salvation is indistinct from the initiation of perpetual friendship with God (cf. Catechism no. 277). According to eternal security, the second birth is the sole moment in which salvation is irrevocably granted, distinct from the initiation of friendship with God. Thus, divine friendship is a "benefit" of salvation, not salvation itself.
Many Protestants who believe that salvation cannot be affected by the state of one's fellowship with God cite Rom. 8:1 as a biblical support: "There is therefore now no condemnation for those who are in Christ Jesus . . ." Yet, St. Paul designates "those who are in Christ" as those "who walk . . . according to the Spirit", not simply who are "born again" (Rom. 8:4). According to the Catholic faith, to "walk in the Spirit" is to persevere in grace.The "Grace Connection" to the Divine Processions
The Catholic dogma of sanctifying grace "connects" Trinitarian dogma to soteriology in the hierarchy of truths. According to the Catholic faith, sanctifying grace is the divine gift that enables human beings to participate in the interior life of God (cf. Catechism no.1997). As Pope Paul VI affirmed in Credo of the People of God, the divine Processions constitute God's eternal Trinitarian life. Therefore mankind's share in the life of God through sanctifying grace is a participation in the divine Processions by which God is triune.
The divine Processions are implicit in the biblical revelation of the Son and the Holy Spirit. Jesus is "the only [begotten] Son who is in the bosom of the Father"; and the Holy Spirit is "the Lord" who "proceeds from the Father," and whom Jesus symbolically "breathed on" His Apostles (Jn. 1:18, 15:26, 20:22; 2 Cor. 3:17). The Magisterium has dogmatically interpreted these and similar passages as revealed analogies between God's interior life, and the basic biological activities of producing offspring and respiration (cf. Jn. 16:13; CCC nos. 249-250). Thus the divine Processions are the eternal activities of God's life: divine Generation is the eternal begetting of the Son by the Father; and divine Spiration is the eternal procession of the Spirit from the Father and the Son (cf. CCC nos. 242, 246).