Why is it presumptuous to assume we are loved by God?
It is presumptuous to assume that we will not yield to temptation and reject God’s love in favor of love for the world or apostasy. From a Calvinist perspective, therefore, if one accepts the perseverance of the saints, the challenge is that one does not know if one is regenerate; from a non-Calvinist perspective the challenge is that one does not know if one will fall away.
There is scriptural backing for either a deterministic or a non-deterministic perspective, but equally strong support in scripture for being wary of a false sense of security concerning our soteriological outcome. For example, the Holy Apostle Paul worried that he might fall alive; the Holy Apostle Peter warns us that our adversary the devil is like a roaring lion, our Lord, God and Savior Jesus Christ warns us of false prophets and false Christs who will deceive, if possible, even the elect, and to that end Paul again warns us to anathematize anyone who comes preaching another Gospel - which the Church in Rome did in the case of Montanus, but Tertullian, who had been a pious Christian, followed Montanus, and this was the major Early Church Father who coined the word “trinitas” from which we get Trinity. Paul also tells us that we must work out our salvation with fear and trembling.
So, by all means, one can, on a purely scriptural basis, believe in the perseverence of the saints* but as
@98cwitr says, “salvation cannot be lost, but it can be misperceived.” This is why we are told to pray that God will forgive us our trespasses, even as we forgive those who have sinned against us. We should pray to our Lord and Savior for His mercy, which He does show in this life and in the next.
*That said, the early church did not interpret 1 Peter as teaching the perseverance of the saints, and was non-Calvinistic and non-OSAS; of the five solas, only sola gratia has a Patristic origin; sola fide, sola scriptura, even to a certain extent sola dei gloria are 16th century concepts, since the early Church understood Salvation as Glorification and rejected monergism formally at the Fifth Ecumenical Council in Constantinople in the early Sixth Century.