I forgot a biblical verse where it is stated that those in heaven, the saints, are also like angels. So like the angel in Rev 8, our prayers of intercession are offered by heavenly saints in the same way. In addition, those in heaven have been cleansed of sin, since no one can enter heaven unless one is clean. So we know those saints are in heaven, and praying and praising the Lord.
The verse you are talking about is in reference to the Sadducees not believing in the Resurrection and trying to trap Jesus by asking Him about a woman who married 7 times and then died. They asked who will be their husband? And the reference is in the time of the resurrection.
Mark 12
25For when they rise from the dead, they neither marry nor are given in marriage, but are like angels in heaven.
All we can derive here is at the time of the resurrection, we do not marry since angels do not marry.
As for entering heaven, that can happens because of what Jesus did on the cross:
Hebrews 10
19 Therefore, brothers, since we have confidence to enter the holy places by the blood of Jesus, 20by the new and living way that he opened for us through the curtain, that is, through his flesh, 21and since we have a great priest over the house of God, 22let us draw near with a true heart in full assurance of faith, with our hearts sprinkled clean from an evil conscience and our bodies washed with pure water. 23 Let us hold fast the confession of our hope without wavering, for he who promised is faithful. 24And let us consider how to stir up one another to love and good works, 25 not neglecting to meet together, as is the habit of some, but encouraging one another, and all the more as you see the Day drawing near.
26For if we go on sinning deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, 27 but a fearful expectation of judgment, and a fury of fire that will consume the adversaries.
Could you supply your biblical references or verse where you state in the context of the verse where the word “saint” refers to those still on earth?
there are so many versus where the word saints is used about people on earth, better for you to do a search on Bible Gateway here is one about saints in Jerusalem.
Acts 9:13
But Ananias answered, "Lord, I have heard from many about this man, how much evil he has done to your saints at Jerusalem.
Jesus referred to saints who had fallen asleep resurrecting. If the word saints meant people who dies, no need to qualify with fallen asleep.
Matthew 27:52
The tombs also were opened. And many bodies of the saints who had fallen asleep were raised,
All you can derive from the word saint is that they are believers. Nothing to do about dead or alive.
The saints mentioned in Rev could mean both, but I have to check commentaries. But we know there are saints already in Heaven, so when we ask for their intercession, we know they are closer to God than we here on earth. But do not forget the fact that when Catholics ask for their intercession, we know that it is as per God’s will and grace to grant it.
We know that people who believed in Christ go to heaven sure, and in a general sense “saints” are on earth and in heaven. There bodies are asleep waiting for the resurrection and a new glorified body.
But our spirits and theirs are in the same condition based on the blood of Christ and faith in his death and resurrection.