First sign: early writers kept the souls of the faithful outside heaven until the resurrection
First, most of the church writers who lived in the second and third centuries after Christ held that the souls of all believers, even the holiest, are kept in certain places outside heaven until the last day of judgment and are not admitted to the heavenly homeland until they receive their bodies back at the resurrection.
Some of them, it is true, give the martyrs a special place by a unique privilege, namely paradise, as they call it. But by that name they by no means mean the highest heaven itself, where the martyrs would see the Lord face to face. They mean a region below heaven, happier and more pleasant, indeed, than Abraham's bosom, where the souls of the other godly rest, yet not capable of heavenly glory. John Daillé, of renowned memory, shows this at length against Cardinals Bellarmine and Perron.
It follows that in those first centuries of Christianity the doctrine of the worship and invocation of saints now prevailing in the Roman Church had not yet been received as apostolic tradition. That worship rests on the assumption that the saints enjoy the highest glory and blessedness. Since, according to the Roman school, the souls of the saints were not admitted to that glory before Christ's coming, they admit that the saints were not yet solemnly worshiped and invoked. (On Punishments and Satisfactions, book 5, chapters 3 to 6; and On the Object of Religious Worship, book 3, chapter 22.)
Latin
LXI. Primo enim, qui secundo & tertio post Christum saeculo vixerunt scriptores Ecclesiastici, in ea plerique fuerunt sententia, omnium fidelium, etiam sanctissimorum, animas certis extra caelum locis, ad supremum judicii diem contineri, neque ante receptum ex resurrectione corpus in patriam illam caelestem admitti. Eorum quidem nonnulli, ex singulari quadam praerogativa, specialem locum Martyribus assignant, paradisum scilicet, ut vocant. Sed eo nomine minime designant caelum ipsum summum, in quo Martyres Dominum videant facie ad faciem: verum regionem quandam subcaelestem, sinu quidem Abrahae, in quo reliquorum piorum animae requiescunt, laetiorem & amoeniorem, non tamen caelestis gloriae capacem: ut adversus Cardinales Bellarminum atque Perronium, fuse demonstrat inclytae memoriae Joannes Dallaeus. Unde patet primis illis Christianismi saeculis, nondum ut traditionem Apostolicam receptum fuisse dogma de illo Sanctorum cultu & invocatione, qui in Ecclesia Romana jam obtinet. Etenim cultus ille nititur hypothesi de Sanctorum gloria & Beatitudine summa. Ad quam cum non admitterentur, juxta Scholam Romanam, Sanctorum animae ante Christi adventum, ideo fatentur illos nondum fuisse solenniter cultos & invocatos. De poenis & satisfactionibus lib. 5. cap. 3. 4. 5. & 6. Et De Objecto cultus Religiosi lib. 3. cap. 22.